Special Relativity

First Edition, 2007

ISBN 978 81 89940 12 6

© All rights reserved.

Published by: Global Media 1819, Bhagirath Palace, Chandni Chowk, Delhi-110 006 Email: [email protected]

Table of Contents 1. The Principle of Relativity 2. Spacetime 3. Dynamics 4. Aether 5. Mathematical Approach 6. Mathematical Approach - 2 7. Mathematical Transformations 8. Mathematical Appendix

The principle of relativity The principle of relativity Principles of relativity address the problem of how events that occur in one place are observed from another place. This problem has been a difficult theoretical challenge since the earliest times. Aristotle argued in his “Physics” that things must either be moved or be at rest. According to Aristotle, on the basis of complex and interesting arguments about the possibility of a ‘void’, things cannot remain in a state of motion without something moving them. As a result Aristotle proposed that objects would stop entirely in empty space. Galileo challenged this idea of movement being due to a continuous action of something that causes the movement. In his “Dialogue Concerning the Two Chief World Systems” he considers observations of motion made by people inside a ship who could not see the outside:

“have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still. “ According to Galileo, if the ship moves smoothly someone inside it would be unable to determine whether they are moving. This concept leads to Galilean Relativity in which it is held that things continue in a state of motion unless acted upon. Galilean Relativity contains two important principles: firstly it is impossible to determine who is actually at rest and secondly things continue in uniform motion unless acted upon. The second principle is known as Galileo’s Law of Inertia or Newton’s First Law of Motion.

Frames of reference, events and transformations Physical observers are considered to be surrounded by a reference frame which is a set of coordinate axes in terms of which position or movement may be specified or with reference to which physical laws may be mathematically stated. An inertial reference frame is a collection of objects that have no net motion relative to each other. It is a coordinate system defined by the nonaccelerated motion of objects with a common direction and speed.

An event is something that happens independently of the reference frame that might be used to describe it. Turning on a light or the collision of two objects would constitute an event. Suppose there is a small event, such as a light being turned on, that is at coordinates x,y,z,t in one reference frame. What coordinates would another observer, in another reference frame moving relative to the first at velocity v along the x axis assign to the event? This problem is illustrated below:

What we are seeking is the relationship between the second observer’s coordinates x’,y’,z’,t’ and the first observer’s coordinates x,y,z,t. According to Newtonian Relativity: x’ = x − vt y’ = y z’ = z t’ = t This set of equations is known as a Galilean coordinate transformation or Galilean transformation. These equations show how the position of an event in one reference frame is related to the position of an event in another reference frame. But what happens if the event is something that is moving? How do velocities transform from one frame to another?

The calculation of velocities depends on Newton’s formula: v = dx / dt. The use of Newtonian physics to calculate velocities and other physical variables has led to Galilean Relativity being called Newtonian Relativity in the case where conclusions are drawn beyond simple changes in coordinates. The velocity transformations for the velocities in the three directions in space are, according to Galilean relativity:

This result is known as the classical velocity addition theorem and summarises the transformation of velocities between two Galilean frames of reference. It means that the velocities of projectiles must be determined relative to the velocity of the source and destination of the projectile. For example, if a sailor throws a stone at 10 km/hr from Galileo’s ship which is moving towards shore at 5 km/hr then the stone will be moving at 15 km/hr when it hits the shore. In Newtonian Relativity the geometry of space is assumed to be Euclidian and the measurement of time is assumed to be the same for all observers. The derivation of the classical velocity addition theorem is as follows: If the Galilean transformations are differentiated with respect to time: x’ = x − vt So: dx’ / dt = dx / dt − v But in Galilean relativity t’ = t and so dx’ / dt’ = dx’ / dt therefore: dx’ / dt’ = dx / dt − v dy’ / dt’ = dy / dt dz’ / dt’ = dy / dt If we write

etc. then:

Special relativity In the nineteenth century James Clerk Maxwell discovered the equations that describe the propagation of electromagnetic waves such as light. For example, one of his equations determines the velocity of light based on the

permittivity and permeability of the medium through which it travels. If one assumes that both the Maxwell equations are valid, and that the Galilean transformation is the appropriate transformation, then it should be possible to measure velocity absolutely and there should be a preferred reference frame. The preferred reference frame could be considered the true zero point to which all velocity measurements could be referred. Special relativity restored a principle of relativity in physics by maintaining that although Maxwell’s equations are correct Galilean relativity is wrong: there is no preferred reference frame. Special relativity brought back the interpretation that in all inertial reference frames the same physics is going on and there is no phenomenon that would allow an observer to pinpoint a zero point of velocity. Einstein extended the principle of relativity by proposing that the laws of physics are the same regardless of inertial frame of reference. According to Einstein, whether you are in the hold of Galileo’s ship or in the cargo bay of a space ship going at a large fraction of the speed of light the laws of physics will be the same.

The postulates of special relativity 1. First postulate: the principle of relativity Observation of physical phenomena by more than one inertial observer must result in agreement between the observers as to the nature of reality. Or, the nature of the universe must not change for an observer if their inertial state changes. Every physical theory should look the same mathematically to every inertial observer. Formally: the laws of physics are the same regardless of inertial frame of reference. 2. Second postulate: invariance of the speed of light The speed of light in vacuum, commonly denoted c, is the same to all inertial observers, is the same in all directions, and does not depend on the velocity of the object emitting the light. Formally: the speed of light in free space is a constant in all inertial frames of reference. Using these postulates Einstein was able to calculate how the observation of events depends upon the relative velocity of observers. He was then able to construct a theory of physics that led to predictions such as the equivalence of mass and energy and early quantum theory.

Spacetime The spacetime interpretation of special relativity Although the special theory of relativity was first proposed by Einstein in 1905, the modern approach to the theory depends upon the concept of a four-dimensional universe, that was first proposed by Hermann Minkowski in 1908, and further developed as a result of the contributions of Emmy Noether. This approach uses the concept of invariance to explore the types of coordinate systems that are required to provide a full physical description of the location and extent of things. The modern theory of special relativity begins with the concept of “length”. In everyday experience, it seems that the length of objects remains the same no matter how they are rotated or moved from place to place. We think that the simple length of a thing is “invariant”. However, as is shown in the illustrations below, what we are actually suggesting is that length seems to be invariant in a three-dimensional coordinate system.

The length of a thing in a two-dimensional coordinate system is given by Pythagoras’s theorem: h2 = x2 + y2

This two-dimensional length is not invariant if the thing is tilted out of the two-dimensional plane. In everyday life, a three-dimensional coordinate system seems to describe the length fully. The length is given by the threedimensional version of Pythagoras’s theorem: h2 = x2 + y2 + z2 The derivation of this formula is shown in the illustration below.

It seems that, provided all the directions in which a thing can be tilted or arranged are represented within a coordinate system, then the coordinate system can fully represent the length of a thing. However, it is clear that things may also be changed over a period of time. We must think of time as another direction in which things can be arranged. This is shown in the following diagram:

The path taken by a thing in both space and time is known as the space-time interval. Hermann Minkowski realised in 1908 that if things could be rearranged in time, then the universe might be four-dimensional. He boldly suggested that Einstein’s recently-discovered theory of Special Relativity was a consequence of this four-dimensional universe. He proposed that the space-time interval might be related to space and time by Pythagoras’ theorem in four dimensions: s2 = x2 + y2 + z2 + (ict)2 Where i is the imaginary unit (sometimes imprecisely called ), c is a constant, and t is the time interval spanned by the space-time interval, s. The symbols x, y and z represent displacements in space along the corresponding axes. In this equation, the ‘second’ becomes just another unit of length. In the same way as centimetres and inches are both units of length related by centimetres = ‘conversion constant’ times inches, metres and seconds are related by metres = ‘conversion constant’ times seconds.

The conversion constant, c has a value of about 300,000,000 meters per second. Now i2 is equal to minus one, so the space-time interval is given by: s2 = x2 + y2 + z2 − (ct)2 Minkowski’s use of the imaginary unit has been superseded by the use of advanced geometry, that uses a tool known as the “metric tensor”, but his original equation survives, and the space-time interval is still given by: s2 = x2 + y2 + z2 − (ct)2 Space-time intervals are difficult to imagine; they extend between one place and time and another place and time, so the velocity of the thing that travels along the interval is already determined for a given observer. If the universe is four-dimensional, then the space-time interval will be invariant, rather than spatial length. Whoever measures a particular spacetime interval will get the same value, no matter how fast they are travelling. The invariance of the space-time interval has some dramatic consequences. The first consequence is the prediction that if a thing is travelling at a velocity of c metres per second, then all observers, no matter how fast they are travelling, will measure the same velocity for the thing. The velocity c will be a universal constant. This is explained below. When an object is travelling at c, the space time interval is zero, this is shown below:

The space-time interval is s2 = x2 + y2 + z2 − (ct)2 The distance travelled by an object moving at velocity v in the x direction for t seconds is: x = vt

If there is no motion in the y or z directions the space-time interval is s2 = x2 + 0 +

0 − (ct)2

So: s2 = (vt)2 − (ct)2 But when the velocity v equals c: s2 = (ct)2 − (ct)2

And hence the space time interval s2 = (ct)2 − (ct)2 = 0 A space-time interval of zero only occurs when the velocity is c. When observers observe something with a space-time interval of zero, they all

observe it to have a velocity of c, no matter how fast they are moving themselves. The universal constant, c, is known for historical reasons as the “speed of light”. In the first decade or two after the formulation of Minkowski’s approach many physicists, although supporting Special Relativity, expected that light might not travel at exactly c, but might travel at very nearly c. There are now few physicists who believe that light does not propagate at c. The second consequence of the invariance of the space-time interval is that clocks will appear to go slower on objects that are moving relative to you. Suppose there are two people, Bill and John, on separate planets that are moving away from each other. John draws a graph of Bill’s motion through space and time. This is shown in the illustration below:

Being on planets, both Bill and John think they are stationary, and just moving through time. John spots that Bill is moving through what John calls space, as well as time, when Bill thinks he is moving through time alone. Bill would also draw the same conclusion about John’s motion. To John, it is as if Bill’s time axis is leaning over in the direction of travel and to Bill, it is as if John’s time axis leans over.

John calculates the length of Bill’s space-time interval as: s2 = (vt)2 − (ct)2

whereas Bill doesn’t think he has travelled in space, so writes:

s2 = (0)2 − (cT)2

The space-time interval, s2, is invariant. It has the same value for all observers, no matter who measures it or how they are moving in a straight line. Bill’s s2 equals John’s s2 so: (0)2 − (cT)2 = (vt)2 − (ct)2

and

− (cT)2 = (vt)2 − (ct)2

hence . So, if John sees Bill measure a time interval of 1 second (T = 1) between two ticks of a clock that is at rest in Bill’s frame, John will find that his own clock measures between these same ticks an interval t, called coordinate time, which is greater than one second. It is said that clocks in motion slow down, relative to those on observers at rest. This is known as “relativistic time dilation of a moving clock”. The time that is measured in the rest frame of the clock (in Bill’s frame) is called the proper time of the clock. John will also observe measuring rods at rest on Bill’s planet to be shorter than his own measuring rods, in the direction of motion. This is a prediction known as “relativistic length contraction of a moving rod”. If the length of a rod at rest on Bill’s planet is X, then we call this quantity the proper length of the rod. The length x of that same rod as measured on John’s planet, is called coordinate length, and given by

. The last consequence is that clocks will appear to be out of phase with each other along the length of a moving object. This means that if one observer sets up a line of clocks that are all synchronised so they all read the same time, then another observer who is moving along the line at high speed will see the clocks all reading different times. In other words observers who are moving relative to each other see different events as simultaneous. This effect is known as Relativistic Phase or the Relativity of Simultaneity. Relativistic phase is often overlooked by students of Special Relativity, but if it is understood then phenomena such as the twin paradox are easier to understand. The way that clocks go out of phase along the line of travel can be calculated from the concepts of the invariance of the space-time interval and length contraction.

The relationship for comparing lengths in the direction of travel is given by:

. So distances between two points according to Bill are simple lengths in space (X) whereas John sees Bill’s measurement of distance as a combination of a distance (x) and a time interval: x2 = X2 − (cT)2

But from :

.

x2 = X2 − (v2 / c2)X2 So: (cT)2 = (v2 / c2)X2 And cT = (v / c)X

So: T = (v / c2)X Clocks that are synchronised for one observer go out of phase along the line of travel for another observer moving at v metres per second by :(v / c2) seconds for every metre. This is one of the most important results of Special Relativity and is often neglected by students. The net effect of the four-dimensional universe is that observers who are in motion relative to you seem to have time coordinates that lean over in the direction of motion, and consider things to be simultaneous, that are not simultaneous for you. Spatial lengths in the direction of travel are shortened, because they tip upwards and downwards, relative to the time axis in the direction of travel, akin to a rotation out of three-dimensional space.

Great care is needed when interpreting space-time diagrams. Diagrams present data in two dimensions, and cannot show faithfully how, for instance, a zero length space-time interval appears.

Spacetime

Spacetime diagram showing an event, a world line, and a line of simultaneity. In order to gain an understanding of both Galilean and Special Relativity it is important to begin thinking of space and time as being different dimensions of a four-dimensional vector space called spacetime. Actually, since we can’t visualize four dimensions very well, it is easiest to start with only one space dimension and the time dimension. The figure shows a graph with time plotted on the vertical axis and the one space dimension plotted on the horizontal axis. An event is something that occurs at a particular time and a particular point in space. (“Julius X. wrecks his car in Lemitar, NM on 21 June at 6:17 PM.”) A world line is a plot of the position of some object as a function of time (more properly, the time of the object as a function of position) on a spacetime diagram. Thus, a world line is really a line in spacetime, while an event is a point in spacetime. A horizontal line parallel to the position axis (x-axis) is a line of simultaneity; in Galilean Relativity all events on this line occur simultaneously for all observers. It will be seen that the line of simultaneity differs between Galilean and Special Relativity; in Special Relativity the line of simultaneity depends on the state of motion of the observer. In a spacetime diagram the slope of a world line has a special meaning. Notice that a vertical world line means that the object it represents does not move—the velocity is zero. If the object moves to the right, then the world line tilts to the right, and the faster it moves, the more the world line tilts. Quantitatively, we say that

(5.1) Notice that this works for negative slopes and velocities as well as positive ones. If the object changes its velocity with time, then the world line is

curved, and the instantaneous velocity at any time is the inverse of the slope of the tangent to the world line at that time. The hardest thing to realize about spacetime diagrams is that they represent the past, present, and future all in one diagram. Thus, spacetime diagrams don’t change with time—the evolution of physical systems is represented by looking at successive horizontal slices in the diagram at successive times. Spacetime diagrams represent the evolution of events, but they don’t evolve themselves.

The lightcone Things that move at the speed of light in our four dimensional universe have surprising properties. If something travels at the speed of light along the xaxis and covers x meters from the origin in t seconds the space-time interval of its path is zero. s2 = x2 − (ct)2 but x = ct so: s2 = (ct)2 − (ct)2 = 0 Extending this result to the general case, if something travels at the speed of light in any direction into or out from the origin it has a space-time interval of 0: 0 = x2 + y2 + z2 − (ct)2 This equation is known as the Minkowski Light Cone Equation. If light were travelling towards the origin then the Light Cone Equation would describe the position and time of emission of all those photons that could be at the origin at a particular instant. If light were travelling away from the origin the equation would describe the position of the photons emitted at a particular instant at any future time ‘t’.

At the superficial level the light cone is easy to interpret. It’s backward surface represents the path of light rays that strike a point observer at an instant and it’s forward surface represents the possible paths of rays emitted from the point observer at an instant (assuming the conditions appropriate to a special relativistic treatment prevail). Things that travel along the surface of the light cone are said to be light- like and the path taken by such things is known as a null geodesic. Events that lie outside the cones are said to be space-like or, better still space separated because their space time interval from the observer has the same sign as space (positive according to the convention used here). Events that lie within the cones are said to be time-like or time separated because their space-time interval has the same sign as time. However, there is more to the light cone than the propagation of light. If the added assumption is made that the speed of light is the maximum possible velocity then events that are space separated cannot affect the observer directly. Events within the backward cone can have affected the observer so the backward cone is known as the “affective past” and the observer can

affect events in the forward cone hence the forward cone is known as the “affective future”. The assumption that the speed of light is the maximum velocity for all communications is neither inherent in nor required by four dimensional geometry although the speed of light is indeed the maximum velocity for objects if the principle of causality is to be preserved by physical theories (ie: that causes precede effects).

The Lorentz transformation equations The discussion so far has involved the comparison of interval measurements (time intervals and space intervals) between two observers. The observers might also want to compare more general sorts of measurement such as the time and position of a single event that is recorded by both of them. The equations that describe how each observer describes the other’s recordings in this circumstance are known as the Lorentz Transformation Equations. (Note that the symbols below signify coordinates.)

The table below shows the Lorentz Transformation Equations.

y’ = y

y = y’

z’ = z

z = z’

Notice how the phase ( (v/c2)x ) is important and how these formulae for absolute time and position of a joint event differ from the formulae for intervals.

More about the relativity of simultaneity and the Andromeda paradox If two observers who are moving relative to each other synchronise their clocks in their own frames of reference they discover that the clocks do not agree between the reference frames. This is illustrated below:

The effect of the relativity of simultaneity, or “phase”, is for each observer to consider that a different set of events is simultaneous. Phase means that observers who are moving relative to each other have different sets of things that are simultaneous, or in their “present moment”.

The amount by which the clocks differ between two observers depends upon the distance of the clock from the observer (t = xv / c2). Notice that if both observers are part of inertial frames of reference with clocks that are synchronised at every point in space then the phase difference can be obtained by simply reading the difference between the clocks at the distant point and clocks at the origin. This difference will have the same value for both observers. Relativistic phase differences have the startling consequence that at distances as large as our separation from nearby galaxies an observer who is driving on the earth can have a radically different set of events in her “present moment” from another person who is standing on the earth. The classic example of this effect of phase is the “Andromeda Paradox”, also known as the “Rietdijk-Putnam-Penrose” argument. Penrose described the argument: “Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made. How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability.” (Penrose 1989).

The argument is illustrated below:

This “paradox” has generated considerable philosophical debate on the nature of time and free-will. A result of the relativity of simultaneity is that if the car driver launches a space rocket towards the Andromeda galaxy it might have a several days head start compared with a space rocket launched from the ground. This is because the “present moment” for the moving car driver is progressively advanced with distance compared with the present moment on the ground. The present moment for the car driver is shown in the illustration below:

The result of the Andromeda paradox is that when someone is moving towards a distant point there are later events at that point than for someone who is not moving towards the distant point. There is a time gap between the events in the present moment of the two people.

The nature of length contraction According to special relativity items such as measuring rods are extended in both space and time. This means that two observers moving relative to each other will usually be observing different three dimensional sections of the universe. If the word “rod” means the three dimensional form of the object called a rod then these two observers in relative motion observe different rods. Each three dimensional section is those events that are at an observer’s present instant. The area of a Minkowski diagram that corresponds to the extension of an object through time is known as the worldtube of the object. It can be seen in the image below that length contraction is the result of observer’s having different sections of an object’s worldtube in their present instant.

(It should be recalled that the longest lengths on space-time diagrams are often the shortest in reality).

De Broglie waves De Broglie noticed that the differing three dimensional sections of the universe would cause oscillations in the rest frame of an observer to appear as wave trains in the rest frame of observers who are moving.

He combined this insight with Einstein’s ideas on the quantisation of energy to create the foundations of quantum theory. De Broglie’s insight is also a round-about proof of the description of length contraction given above observers in relative motion have differing three dimensional slices of a four dimensional universe. The existence of matter waves is direct experimental evidence of the relativity of simultaneity.

More about time dilation The term “time dilation” is applied to the way that observers who are moving relative to you record fewer clock ticks between events than you. In special relativity this is not due to properties of the clocks, it is due to shorter distances between events along the time axis. This can be seen most clearly by re-examining the Andromeda Paradox. Suppose Bill passes Jim at high velocity on the way to Mars. Jim has previously synchronised the clocks on Mars with his Earth clocks but for Bill the Martian clocks read times well in advance of Jim’s. This means that Bill has a head start because his present instant contains what Jim considers to be the Martian future. Jim observes that Bill travels through both space and time. However, Bill achieves this strange time travel by bringing the

future of distant objects into his present moment. Bill is literally travelling into future parts of Jim’s frame of reference.

The twin paradox The “Andromeda paradox” is, in part, the origin of the “twin paradox”. In the twin paradox there are twins, Bill and Jim. Jim is on Earth. Bill flies past Jim in a spaceship, goes to a distant point, turns round and flies back again. It is found that Bill records fewer clock ticks over the whole journey than Jim records on earth. Why? Suppose Jim has synchronised clocks on Earth and on the distant point. As Bill flies past Jim he synchronises his clock with Jim’s clock. When he does this he observes the clocks on the distant point and immediately detects that they are not synchronised with his or Jim’s clocks. To Bill it appears that Jim has synchronised his clocks incorrectly. There is a time difference, or “gap”, between his clocks and those at the distant point even when he passes Jim. This difference is equal to the relativistic phase at the distant point. Bill flies to the distant point and discovers that the clock there is reading a later time than his own clock. He turns round to fly back to Earth and observes that the clocks on Earth seem to have jumped forward, yet another “time gap” appears. When Bill gets back to Earth the time gaps and time dilations mean that people on Earth have recorded more clock ticks that he did. For ease of calculation suppose that Bill is moving at a truly astonishing velocity of 0.8c in the direction of a distant point that is 10 light seconds away (about 3 million kilometres). The illustration below shows Jim and Bill’s observations:

From Bill’s viewpoint there is both a time dilation and a phase effect. It is the added factor of “phase” that explains why, although the time dilation occurs for both observers, Bill observes the same readings on Jim’s clocks over the whole journey as does Jim. To summarise the mathematics of the twin paradox using the example:

Jim observes the distance as 10 light seconds and the distant point is in his frame of reference. According to Jim it takes Bill the following time to make the journey:

Time taken = distance / velocity therefore according to Jim: t = 10 / 0.8 = 12.5 seconds

Again according to Jim, time dilation should affect the observed time on Bill’s clocks: so:

seconds So for Jim the round trip takes 25 secs and Bill’s clock reads 15 secs. Bill measures the distance as:

light seconds. For Bill it takes X / v = 6 / 0.8 = 7.5 seconds. Bill observes Jim’s clocks to appear to run slow as a result of time dilation:

so:

seconds But there is also a time gap of vx / c2 = 8 seconds. So for Bill, Jim’s clocks register 12.5 secs have passed from the start to the distant point. This is composed of 4.5 secs elapsing on Jim’s clocks plus an 8 sec time gap from the start of the journey. Bill sees 25 secs total time recorded on Jim’s clocks over the whole journey, this is the same time as Jim observes on his own clocks. It is sometimes dubiously asserted that the twin paradox is about the clocks on the twin that leaves earth being slower than those on the twin that stays at home, it is then argued that biological processes contain clocks therefore the twin that travelled away ages less. This is not really true because the relativistic phase plays a major role in the twin paradox and leads to Bill travelling to a remote place that, for Bill, is at a later time than Jim when Bill and Jim pass each other. A more accurate explanation is that when we travel we travel in time as well as space.

The turn around is not required to demonstrate the twin “paradox”. Suppose there were two travellers, Bill(1) who moves away from earth and Bill(2) who travels towards earth. If Bill(2) synchronises his clocks with the clocks on Bill(1) when they pass then the same difference in elapsed time between the clocks on Jim and Bill(2) will be observed as between Jim and Bill in the original example. Students have difficulty with the twin paradox because they believe that the observations of the twins are symmetrical. This is not the case. As can be seen from the illustration below either twin could determine whether they had made the turn or the other twin had made the turn.

Jim and Bill’s view of the journey Special relativity does not postulate that all motion is ‘relative’; the postulates are that the laws of physics are the same in all inertial frames and there is a constant velocity called the “speed of light”. Contrary to popular myth the twins do not observe events that are a mirror image of each other. Bill observes himself leave Jim then return, Jim sees Bill leave him then return. Bill does not observe Jim turn round, he observes himself making the turn. The following illustrations cover various views of the journey. The most important moment in the journey is the point where Bill turns round. Notice

how Bill’s surface of simultaneity, that includes the events that he considers to be in the present moment, swings across Jim’s worldline during the turn.

As Bill travels away from Jim he considers events that are already in Jim’s past to be in his own present.

After the turn Bill considers events that are in Jim’s future to be in his present (although the finite speed of light prevents Bill from observing Jim’s future).

The swing in Bill’s surface of simultaneity at the turn-round point leads to a ‘time gap’. In our example Bill might surmise that Jim’s clocks jump by 16 seconds on the turn.

Notice that the term “Jim’s apparent path” is used in the illustration - as was seen earlier, Bill knows that he himself has left Jim and returned so he knows that Jim’s apparent path is an artefact of his own motion. If we imagine that the twin paradox is symmetrical then the illustration above shows how we might imagine Bill would view the journey. But what happens, in our example, to the 16 seconds in the time gap, does it just disappear? The twin paradox is not symmetrical and Jim does not make a sudden turn after 4.5 seconds. Bill’s actual observation and the fate of the information in the time gap can be probed by supposing that Jim emits a pulse of light several times a second. The result is shown in the illustration below.

Jim has clearly but one inertial frame but does Bill represent a single inertial frame? Suppose Bill was on a planet as he passed Jim and flew back to Jim in a rocket from the turn-round point: how many inertial frames would be involved? Is Bill’s view a view from a single inertial frame? Exercise: it is interesting to calculate the observations made by an observer who continues in the direction of the outward leg of Bill’s journey - note that a velocity transformation will be needed to estimate Bill’s inbound velocity as measured by this third observer.

The Pole-barn paradox

(Note that Minkowski’s metric involves the subtraction of displacements in time, so what appear to be the longest lengths on a 2D sheet of paper are often the shortest lengths in a (3+1)D reality). The symmetry of length contraction leads to two questions. Firstly, how can a succession of events be observed as simultaneous events by another observer? This question led to the concept of de Broglie waves and quantum theory. Secondly, if a rod is simultaneously between two points in one frame how can it be observed as being successively between those points in another frame? For instance, if a pole enters a building at high speed how can one observer find it is fully within the building and another find that the two ends of the rod are opposed to the two ends of the building at successive times? What happens if the rod hits the end of the building? The second question is known as the “pole-barn paradox” or “ladder paradox”. The pole-barn paradox states the following: suppose a superhero running at 0.75c and carrying a horizontal pole 15 m long towards a barn 10m long, with front and rear doors. When the runner and the pole are inside the barn, a ground observer closes and then opens both doors (by remote control) so that the runner and pole are momentarily captured inside the barn and then proceed to exit the barn from the back door.

One may be surprised to see a 15-m pole fit inside a 10-m barn. But the pole is in motion with respect to the ground observer, who measures the pole to be contracted to a length of 9.9 m (check using equations). The “paradox” arises when we consider the runner’s point of view. The runner sees the barn contracted to 6.6 m. Because the pole is in the rest frame of the runner, the runner measures it to have its proper length of 15 m. Now, how can our superhero make it safely through the barn? The resolution of the “paradox” lies in the relativity of simultaneity. The closing of the two doors is measured to be simultaneous by the ground observer. However, since the doors are at different positions, the runner says that they do not close simultaneously. The rear door closes and then opens first, allowing the leading edge of the pole to exit. The front door of the barn does not close until the trailing edge of the pole passes by. The following explanation is taken from Wikipedia. If a long ladder travels horizontally at almost the speed of light, it will undergo a length contraction and will therefore fit into a garage which is shorter than the ladder’s length at rest. On the other hand, from the point of view of the ladder, it is the garage which is moving and its length will be contracted. The garage will therefore need to be larger than the length at rest of the ladder in order to contain it.

Figure 1a: The problem, part 1 - A length contracted ladder being contained within a garage

Figure 1b: The problem, part 2 - A length contracted garage being too small to contain the same long ladder

Relative simultaneity

Figure 1c: The solution from the garage point of view: A length contracted ladder entering and exiting a two-door garage The solution to this dilemma lies in the fact that what one observer (e.g. the garage) considers as simultaneous does not correspond to what the other observer (e.g. the ladder) considers as simultaneous (relative simultaneity). A clear way of seeing this is to consider a garage with two doors that swing shut to contain the ladder and then open again to let the ladder out the other side.

Figure 1d: The solution from the ladder point of view: A length contracted two-door garage passing over a ladder From the perspective of the ladder what happens is that first one door closes and opens and then, after the garage passes over the ladder, the second door closes and opens. The situation is illustrated in the Minkowski diagram below. The diagram is in the rest frame of the garage. The vertical blue band shows the garage in space-time, the red band shows the ladder in space-time. The x and t axes are the garage space and time axes, respectively, and x′ and t′ are the ladder space and time axes, respectively. The ladder is moving at a velocity of in the positive x direction, therefore . Since light travels at very close to one foot per nanosecond, let’s work in these units, so that c = 1ft/ns. The garage is a small one, G=10 feet long, while the ladder is L=12 feet long. In the garage frame, the front of the ladder will hit the back of the garage at time . This is shown as event A on the diagram. All lines parallel to the garage x axis will be simultaneous according to the garage observer, so the blue line AB will be what the garage observer sees as the ladder at the time of event A. The ladder is contained inside the garage. However, from the point of view of the observer on the ladder, the red line AC is what the ladder observer sees as the ladder. The back of the ladder is outside the garage.

Figure 1e: A Minkowski diagram of ladder paradox No. 1. The garage is shown in blue, the ladder in red. The diagram is in the rest frame of the garage, with x and t being the garage space and time axes, respectively. The ladder frame is for a person sitting on the front of the ladder, with x′ and t′ being the ladder space and time axes respectively. The x and x′ axes are each 5 feet long, and the t and t′ axes are each 5 ns in duration.

Using the one-dimensional Lorentz transformation we can get:

Event

Garage Frame

Ladder Frame

Relativistic trains passing Another dramatisation of the same effect is to consider two relativistic trains approaching each other along a single track, with only a short length of double track for the trains to pass, at a station platform. The platform is shorter than the rest length of the trains (just as the garage is shorter than the ladder); but the trains can just pass (from the perspective of an observer on the platform), thanks to the relativistic length contraction: the head of each train reaches the end of the platform just as the tail of the other train clears it, going in the other direction. From the train perspective, it is now the platform which is contracted. As before, the nose of the train reaches the end of the platform just as the tail of the other train clears it. However, in this perspective, the train still has its full rest-length. So the tail of the first train is still in the section of singletrack - the front of the platform has not yet reached it. But there is no collision, because the oncoming second train is even more length contracted than the platform, and so its nose has not yet reached the front of the platform. There is just time for tail of the first train to clear the front of the platform, as the nose of the second train reaches it. So a crash is again averted, in this frame because of the relativity of simultaneity: what is simultaneous to an observer standing on the platform is not simultaneous for an observer on one of the trains.

Resolution of the paradox if the ladder stops

A ladder contracting under acceleration to fit into a length contracted garage When the stationary garage traps the moving ladder, what happens after the event is either 1)the ladder continues out the other side of the garage (the above two-door garage example) or 2)the ladder comes to a complete halt within the garage. Considering just the latter, we can say that every point of

the ladder is simultaneously at rest from the perspective of the garage. From the perspective of the ladder this cannot be true. What the ladder experiences is one end decelerating in order to catch up to the garage, then the next point of the ladder decelerating, followed by the next point, and so on until finally the entire ladder decelerates. In other words, the ladder contracts under its own deceleration as it suddenly decelerates to catch up to the garage (enter the inertial reference frame of the garage). And so from both perspectives the garage manages to contain the ladder. (After that, since it is no longer moving in relation to the garage, the ladder may revert back to its original length, possibly causing damage to itself and the garage.)

Figure 1 - A Minkowski diagram of the case where the ladder is stopped all along its length, simultaneously in the garage frame. When this occurs, the garage frame sees the ladder as AB, but the ladder frame sees the ladder as AC. When back of the ladder enters the garage at point D, it has not yet felt the effects of the deceleration. At this time, according to someone at rest with respect to the back of the ladder, the front of the ladder will be at point E and will see the ladder as DE. It is seen that this length in the ladder frame is not the same as CA, the rest length of the ladder before the deceleration.

Man falling into grate variation

A man (represented by a segmented rod) falling into a grate This paradox was originally proposed and solved by Wolfgang Rindler (“Length Contraction Paradox”: Am. J. Phys., 29(6) June 1961) and involved a fast walking man, represented by a rod, falling into a grate. From the perspective of the grate the man undergoes a length contraction and falls into the grate. However from the perspective of the man it is the grate undergoing a length contraction and seems too small for the man to fall through. Using the solution of relative simultaneity, we can see that from the perspective of the grate every part of the man falls into the grate at the same time while from the perspective of the man his front falls into the grate before his rear, as can be easily seen if the man is represented by a segmented rod. This causes him to bend into the grate, which is similar to the ladder contracting to fit into the garage. In fact, if the man hits a bar of the grate as he falls in, since he is travelling forwards as well as falling and hitting the bar brings him to a halt, he undergoes the same contraction that the ladder experiences as well as bending into the grate.

Recent criticism

However, the above “man falling into grate” solution has recently been criticised in an article by Van Lintel and Gruber in the Eur.J.Phys.26 (Jan. 2005), p.19: the above solution implied that proper stiffness could be affected by relative speed. The new solution is that the rod’s (unaffected) proper stiffness is related to the thickness of the rod, which thickness implies a time delay before any part of the upper surface of the rod can start falling. In this particular case, the rod will even arrive at the other side of the hole before any part of the upper surface “feels” the effect of the hole. In other words, the above pictures are misleading: the correct solution is that the rod remains as straight (stiff) as one would expect, and that therefore the rod doesn’t fall into the hole.

Addition of velocities How can two observers, moving at v km/sec relative to each other, compare their observations of the velocity of a third object?

Suppose one of the observers measures the velocity of the object as u’ where:

The coordinates x’ and t’ are given by the Lorentz transformations:

and

but x’ = u’t’ so:

and hence: x − vt = u’(t − vx / c2) Notice the role of the phase term vx / c2. The equation can be rearranged as:

given that x = ut:

This is known as the relativistic velocity addition theorem, it applies to velocities parallel to the direction of mutual motion. The existence of time dilation means that even when objects are moving perpendicular to the direction of motion there is a discrepancy between the velocities reported for an object by observers who are moving relative to each other. If there is any component of velocity in the x direction (ux, ) then the phase affects time measurement and hence the velocities

perpendicular to the x-axis. The table below summarises the relativistic addition of velocities in the various directions in space.

Notice that for an observer in another reference frame the sum of two velocities (u and v) can never exceed the speed of light.

Dynamics Introduction The way that the velocity of a particle can differ between observers who are moving relative to each other means that momentum needs to be redefined as a result of relativity theory. The illustration below shows a typical collision of two particles. In the right hand frame the collision is observed from the viewpoint of someone moving at the same velocity as one of the particles, in the left hand frame it is observed by someone moving at a velocity that is intermediate between those of the particles.

If momentum is redefined then all the variables such as force (rate of change of momentum), energy etc. will become redefined and relativity will lead to an entirely new physics. The new physics has an effect at the ordinary level of experience through the relation E = mc2 whereby it is the tiny changes in relativistic mass that are expressed as everyday kinetic energy so that the whole of physics is related to "relativistic" reasoning rather than Newton's empirical ideas.

Momentum In physics momentum is conserved within a closed system, the law of conservation of momentum applies. Consider the special case of identical particles colliding symmetrically as illustrated below:

The momentum change by the red ball is:

The momentum change by the blue ball is:

The situation is symmetrical so the Newtonian conservation of momentum law is demonstrated:

Notice that this result depends upon the y components of the velocities being equal ie: . The relativistic case is rather different. The collision is illustrated below, the left hand frame shows the collision as it appears for one observer and the right hand frame shows exactly the same collision as it appears for another observer moving at the same velocity as the blue ball:

The configuration shown above has been simplified because one frame contains a stationary blue ball (ie: uxB = 0) and the velocities are chosen so that the vertical velocity of the red ball is exactly reversed after the collision ie: . Both frames show exactly the same event, it is only the observers who differ between frames. The relativistic velocity transformations between frames is:

given that uxB = 0. Suppose that the y components are equal in one frame, in Newtonian physics they will also be equal in the other frame. However, in relativity, if the y components are equal in one frame they are not necessarily equal in the other frame. For instance if

So if

then in this case

then:

.

If the mass were constant between collisions and between frames then although

it is found that:

So momentum defined as mass times velocity is not conserved in a collision when the collision is described in frames moving relative to each other. Notice that the discrepancy is very small if uxR and v are small. To preserve the principle of momentum conservation in all inertial reference frames, the definition of momentum has to be changed. The new definition must reduce to the Newtonian expression when objects move at speeds much smaller than the speed of light, so as to recover the Newtonian formulas. The velocities in the y direction are related by the following equation when the observer is travelling at the same velocity as the blue ball ie: when uxB = 0:

If we write mB for the mass of the blue ball) and mR for the mass of the red ball as observed from the frame of the blue ball then, if the principle of relativity applies: 2mRuyR = 2mBuyB So:

But:

Therefore:

This means that, if the principle of relativity is to apply then the mass must change by the amount shown in the equation above for the conservation of momentum law to be true. The reference frame was chosen so that This allows v to be determined in terms of uxR:

and hence

.

and hence:

So substituting for v in

:

The blue ball is at rest so its mass is sometimes known as its rest mass, and is given the symbol m0. As the balls were identical at the start of the boost the mass of the red ball is the mass that a blue ball would have if it were in motion relative to an observer; this mass is sometimes known as the relativistic mass symbolised by m. These terms are now infrequently used in modern physics, as will be explained at the end of this section. The discussion given above was related to the relative motions of the blue and red balls, as a result uxR corresponds to the speed of the moving ball relative to an observer who is stationary with respect to the blue ball. These considerations mean that the relativistic mass is given by:

The relativistic momentum is given by the product of the relativistic mass and the velocity . The overall expression for momentum in terms of rest mass is:

and the components of the momentum are:

So the components of the momentum depend upon the appropriate velocity component and the speed. Since the factor with the square root is cumbersome to write, the following abbreviation is often used, called the Lorentz gamma factor:

The expression for the momentum then reads

.

It can be seen from the discussion above that we can write the momentum of an object moving with velocity as the product of a function m(u) of the speed u and the velocity :

The function m(u) must reduce to the object's mass m at small speeds, in particular when the object is at rest m(0) = m. The function m(u) used to be called 'relativistic mass', and its value in the frame of the particle was referred to as the 'rest mass' or 'invariant mass'. Both terms are now obsolete: the 'rest mass' is today simply called the mass, and the 'relativistic mass' is no longer used since, as will be seen in the discussion of energy below, it is identical to the energy but for the units.

Force Newton's second law states that the total force acting on a particle equals the rate of change of its momentum. The same form of Newton's second law holds in relativistic mechanics.

If the relativistic momentum is used:

By Leibniz's law where d(xy) = xdy + ydx:

This equation for force will be used below to derive relativistic expressions for the energy of a particle. In Newtonian mechanics, momentum is can be written in the more familiar form

where

, and Newton's second law

is the acceleration.

In relativistic mechanics, momentum is instead . As will been seen later, when the transformation of acceleration is considered, Newton's second law in terms of acceleration contains extra terms

Energy Energy is defined as the work done in moving a body from one place to another. Energy is given from:

so, over the whole path:

Kinetic energy (K) is the energy used to move a body from a velocity of 0 to a velocity . Restricting the motion to one dimension:

Using the relativistic force:

So:

substituting for d(γu) and using dx / dt = u:

Which gives:

The Lorentz factor γ is given by:

which can be expanded as: γ2c2 − γ2u2 = c2 Differentiating: 2γc2dγ − γ22udu − u22γdγ = 0 So, rearranging: γudu + u2dγ = c2dγ In which case:

is simplified to:

As u goes from 0 to u, the Lorentz factor γ goes from 1 to γ, so:

and hence: K = γmc2 − mc2 The amount γmc2 is known as the total energy of the particle. The amount mc2 is known as the rest energy of the particle. If the total energy of the particle is given the symbol E: E = γmc2 = mc2 + K

So it can be seen that mc2 is the energy of a mass that is stationary. This energy is known as mass energy and is the origin of the famous formula E = mc2 that is iconic of the nuclear age. The Newtonian approximation for kinetic energy can be derived by using the binomial theorem to expand

.

The binomial theorem is:

So expanding

So if u is much less than c:

:

which is the Newtonian approximation for low velocities.

Aether Introduction Many students confuse Relativity Theory with a theory about the propagation of light. According to modern Relativity Theory the constancy of the speed of light is a consequence of the geometry of spacetime rather than something specifically due to the properties of photons; but the statement “the speed of light is constant” often distracts the student into a consideration of light propagation. This confusion is amplified by the importance assigned to interferometry experiments, such as the Michelson-Morley experiment, in most textbooks on Relativity Theory. The history of theories of the propagation of light is an interesting topic in physics and was indeed important in the early days of Relativity Theory. In the seventeenth century two competing theories of light propagation were developed. Christiaan Huygens published a wave theory of light which was based on Huygen’s principle whereby every point in a wavelike disturbance can give rise to further disturbances that spread out spherically. In contrast Newton considered that the propagation of light was due to the passage of small particles or “corpuscles” from the source to the illuminated object. His theory is known as the corpuscular theory of light. Newton’s theory was widely accepted until the nineteenth century. In the early nineteenth century Thomas Young performed his Young’s slits experiment and the interference pattern that occurred was explained in terms of diffraction due to the wave nature of light. The wave theory was accepted generally until the twentieth century when quantum theory confirmed that light had a corpuscular nature and that Huygen’s principle could not be applied. The idea of light as a disturbance of some medium, or aether, that permeates the universe was problematical from its inception (US spelling: “ether”). The first problem that arose was that the speed of light did not change with the velocity of the observer. If light were indeed a disturbance of some stationary medium then as the earth moves through the medium towards a light source the speed of light should appear to increase. It was found however that the speed of light did not change as expected. Each experiment on the velocity of light required corrections to existing theory and led to a variety of subsidiary theories such as the “aether drag hypothesis”. Ultimately it was experiments that were designed to investigate the properties of the aether that provided the first experimental evidence for Relativity Theory.

The aether drag hypothesis

The aether drag hypothesis was an early attempt to explain the way experiments such as Arago’s experiment showed that the speed of light is constant. The aether drag hypothesis is now considered to be incorrect by mainstream science. According to the aether drag hypothesis light propagates in a special medium, the aether, that remains attached to things as they move. If this is the case then, no matter how fast the earth moves around the sun or rotates on its axis, light on the surface of the earth would travel at a constant velocity. The primary reason the aether drag hypothesis is considered invalid is because of the occurrence of stellar aberration. In stellar aberration the position of a star when viewed with a telescope swings each side of a central position by about 20.5 seconds of arc every six months. This amount of swing is the amount expected when considering the speed of earth’s travel in its orbit. In 1871 George Biddell Airy demonstrated that stellar aberration occurs even when a telesope is filled with water. It seems that if the aether drag hypothesis were true then stellar aberration would not occur because the light would be travelling in the aether which would be moving along with the telescope.

If you visualize a bucket on a train about to enter a tunnel and a drop of water drips from the tunnel entrance into the bucket at the very center, the drop will not hit the center at the bottom of the bucket. The bucket is the tube of a telescope, the drop is a photon and the train is the earth. If aether

is dragged then the droplet would be traveling with the train when it is dropped and would hit the center of bucket at the bottom. The amount of stellar aberration, α is given by: tan(α) = vδt / cδt So: tan(α) = v / c The speed at which the earth goes round the sun, v = 30 km/s, and the speed of light is c = 300,000,000 m/s which gives α = 20.5 seconds of arc every six months. This amount of aberration is observed and this contradicts the aether drag hypothesis. In 1818 Fresnel introduced a modification to the aether drag hypothesis that only applies to the interface between media. This was accepted during much of the nineteenth century but has now been replaced by special theory of relativity (see below). The aether drag hypothesis is historically important because it was one of the reasons why Newton’s corpuscular theory of light was replaced by the wave theory and it is used in early explanations of light propagation without relativity theory. It originated as a result of early attempts to measure the speed of light. In 1810 François Arago realised that variations in the refractive index of a substance predicted by the corpuscular theory would provide a useful method for measuring the velocity of light. These predictions arose because the refractive index of a substance such as glass depends on the ratio of the velocities of light in air and in the glass. Arago attempted to measure the extent to which corpuscles of light would be refracted by a glass prism at the front of a telescope. He expected that there would be a range of different angles of refraction due to the variety of different velocities of the stars and the motion of the earth at different times of the day and year. Contrary to this expectation he found that that there was no difference in refraction between stars, between times of day or between seasons. All Arago observed was ordinary stellar aberration. In 1818 Augustin Jean Fresnel examined Arago’s results using a wave theory of light. He realised that even if light were transmitted as waves the refractive index of the glass-air interface should have varied as the glass moved through the aether to strike the incoming waves at different velocities when the earth rotated and the seasons changed.

Fresnel proposed that the glass prism would carry some of the aether along with it so that “..the aether is in excess inside the prism”. He realised that the velocity of propagation of waves depends on the density of the medium so proposed that the velocity of light in the prism would need to be adjusted by an amount of ‘drag’. The velocity of light vn in the glass without any adjustment is given by: vn = c / n The drag adjustment vd is given by:

Where ρe is the aether density in the environment, ρg is the aether density in the glass and v is the velocity of the prism with respect to the aether.

The factor can be written as because the refractive index, n, would be dependent on the density of the aether. This is known as the Fresnel drag coefficient. The velocity of light in the glass is then given by:

This correction was successful in explaining the null result of Arago’s experiment. It introduces the concept of a largely stationary aether that is dragged by substances such as glass but not by air. Its success favoured the wave theory of light over the previous corpuscular theory. The Fresnel drag coefficient was confirmed by an interferometer experiment performed by Fizeau. Water was passed at high speed along two glass tubes that formed the optical paths of the interferometer and it was found that the fringe shifts were as predicted by the drag coefficient.

The special theory of relativity predicts the result of the Fizeau experiment from the velocity addition theorem without any need for an aether. If V is the velocity of light relative to the Fizeau apparatus and U is the velocity of light relative to the water and v is the velocity of the water:

which, if v/c is small can be expanded using the binomial expansion to become:

This is identical to Fresnel’s equation. It may appear as if Fresnel’s analysis can be substituted for the relativistic approach, however, more recent work has shown that Fresnel’s assumptions

should lead to different amounts of aether drag for different frequencies of light and violate Snell’s law (see Ferraro and Sforza (2005)). The aether drag hypothesis was one of the arguments used in an attempt to explain the Michelson-Morley experiment before the widespread acceptance of the special theory of relativity. The Fizeau experiment is consistent with relativity and approximately consistent with each individual body, such as prisms, lenses etc. dragging its own aether with it. This contradicts some modified versions of the aether drag hypothesis that argue that aether drag may happen on a global (or larger) scale and stellar aberration is merely transferred into the entrained “bubble” around the earth which then faithfully carries the modified angle of incidence directly to the observer.

The Michelson-Morley experiment The Michelson-Morley experiment, one of the most important and famous experiments in the history of physics, was performed in 1887 by Albert Michelson and Edward Morley at what is now Case Western Reserve University, and is considered to be the first strong evidence against the theory of a luminiferous aether. Physics theories of the late 19th century postulated that, just as water waves must have a medium to move across (water), and audible sound waves require a medium to move through (air), so also light waves require a medium, the “luminiferous aether”. The speed of light being so great, designing an experiment to detect the presence and properties of this aether took considerable thought.

Measuring aether

A depiction of the concept of the “aether wind”. Each year, the Earth travels a tremendous distance in its orbit around the sun, at a speed of around 30 km/second, over 100,000 km per hour. It was

reasoned that the Earth would at all times be moving through the aether and producing a detectable “aether wind”. At any given point on the Earth’s surface, the magnitude and direction of the wind would vary with time of day and season. By analysing the effective wind at various different times, it should be possible to separate out components due to motion of the Earth relative to the Solar System from any due to the overall motion of that system. The effect of the aether wind on light waves would be like the effect of wind on sound waves. Sound waves travel at a constant speed relative to the medium that they are travelling through (this varies depending on the pressure, temperature etc (see sound), but is typically around 340 m/s). So, if the speed of sound in our conditions is 340 m/s, when there is a 10 m/s wind relative to the ground, into the wind it will appear that sound is travelling at 330 m/s (340 - 10). Downwind, it will appear that sound is travelling at 350 m/s (340 + 10). Measuring the speed of sound compared to the ground in different directions will therefore enable us to calculate the speed of the air relative to the ground. If the speed of the sound cannot be directly measured, an alternative method is to measure the time that the sound takes to bounce off of a reflector and return to the origin. This is done parallel to the wind and perpendicular (since the direction of the wind is unknown before hand, just determine the time for several different directions). The cumulative round trip effects of the wind in the two orientations slightly favors the sound travelling at right angles to it. Similarly, the effect of an aether wind on a beam of light would be for the beam to take slightly longer to travel round-trip in the direction parallel to the “wind” than to travel the same round-trip distance at right angles to it. “Slightly” is key, in that, over a distance such as a few meters, the difference in time for the two round trips would be only about a millionth of a millionth of a second. At this point the only truly accurate measurements of the speed of light were those carried out by Albert Abraham Michelson, which had resulted in measurements accurate to a few meters per second. While a stunning achievement in its own right, this was certainly not nearly enough accuracy to be able to detect the aether.

The experiments Michelson, though, had already seen a solution to this problem. His design, later known as an interferometer, sent a single source of white light through a half-silvered mirror that was used to split it into two beams travelling at right angles to one another. After leaving the splitter, the beams travelled out to the ends of long arms where they were reflected back into the middle on small mirrors. They then recombined on the far side of the splitter in an eyepiece, producing a pattern of constructive and destructive interference based on the length of the arms. Any slight change in the amount of time the beams spent in transit would then be observed as a shift in the positions of

the interference fringes. If the aether were stationary relative to the sun, then the Earth’s motion would produce a shift of about 0.04 fringes. Michelson had made several measurements with an experimental device in 1881, in which he noticed that the expected shift of 0.04 was not seen, and a smaller shift of about 0.02 was. However his apparatus was a prototype, and had experimental errors far too large to say anything about the aether wind. For a measurement of the aether wind, a much more accurate and tightly controlled experiment would have to be carried out. The prototype was, however, successful in demonstrating that the basic method was feasible.

A Michelson interferometer He then combined forces with Edward Morley and spent a considerable amount of time and money creating an improved version with more than enough accuracy to detect the drift. In their experiment the light was repeatedly reflected back and forth along the arms, increasing the path length to 11m. At this length the drift would be about .4 fringes. To make that easily detectable the apparatus was located in a closed room in the basement of a stone building, eliminating most thermal and vibrational effects. Vibrations were further reduced by building the apparatus on top of a huge block of marble, which was then floated in a pool of mercury. They calculated that effects of about 1/100th of a fringe would be detectable. The mercury pool allowed the device to be turned, so that it could be rotated through the entire range of possible angles to the “aether wind”. Even over a short period of time some sort of effect would be noticed simply by rotating the device, such that one arm rotated into the direction of the wind and the other away. Over longer periods day/night cycles or yearly cycles would also be easily measurable. During each full rotation of the device, each arm would be parallel to the wind twice (facing into and away from the wind) and perpendicular to the

wind twice. This effect would show readings in a sine wave formation with two peaks and two troughs. Additionally if the wind was only from the earth’s orbit around the sun, the wind would fully change directions east/west during a 12 hour period. In this ideal conceptualization, the sine wave of day/night readings would be in opposite phase. Because it was assumed that the motion of the solar system would cause an additional component to the wind, the yearly cycles would be detectable as an alteration of the maginitude of the wind. An example of this effect is a helicopter flying forward. While on the ground, a helicopter’s blades would be measured as travelling around at 50 MPH at the tips. However, if the helicopter is travelling forward at 50 MPH, there are points at which the tips of the blades are travelling 0 MPH and 100 MPH with respect to the air they are travelling through. This increases the magnitude of the lift on one side and decreases it on the other just as it would increase and decrease the magnitude of an ether wind on a yearly basis.

The most famous failed experiment Ironically, after all this thought and preparation, the experiment became what might be called the most famous failed experiment to date. Instead of providing insight into the properties of the aether, Michelson and Morley’s 1887 article in the American Journal of Science reported the measurement to be as small as one-fortieth of the expected displacement but “since the displacement is proportional to the square of the velocity” they concluded that the measured velocity was approximately one-sixth of the expected velocity of the Earth’s motion in orbit and “certainly less than one-fourth”. Although this small “velocity” was measured, it was considered far too small to be used as evidence of aether, it was later said to be within the range of an experimental error that would allow the speed to actually be zero. Although Michelson and Morley went on to different experiments after their first publication in 1887, both remained active in the field. Other versions of the experiment were carried out with increasing sophistication. Kennedy and Illingsworth both modified the mirrors to include a half-wave “step”, eliminating the possibility of some sort of standing wave pattern within the apparatus. Illingsworth could detect changes on the order of 1/300th of a fringe, Kennedy up to 1/1500th. Miller later built a non-magnetic device to eliminate magnetostriction, while Michelson built one of non-expanding invar to eliminate any remaining thermal effects. Others from around the world increased accuracy, eliminated possible side effects, or both. All of these with the exception of Dayton Miller also returned what is considered a null result. Morley was not convinced of his own results, and went on to conduct additional experiments with Dayton Miller. Miller worked on increasingly large experiments, culminating in one with a 32m (effective) arm length at an installation at the Mount Wilson observatory. To avoid the possibility of the aether wind being blocked by solid walls, he used a special shed with thin

walls, mainly of canvas. He consistently measured a small positive effect that varied, as expected, with each rotation of the device, the sidereal day and on a yearly basis. The low magnitude of the results he attributed to aether entrainment (see below). His measurements amounted to only ~10 kps instead of the expected ~30 kps expected from the earth’s orbital motion alone. He remained convinced this was due to partial entrainment, though he did not attempt a detailed explanation. Though Kennedy later also carried out an experiment at Mount Wilson, finding 1/10 the drift measured by Miller, and no seasonal effects, Miller’s findings were considered important at the time, and were discussed by Michelson, Hendrik Lorentz and others at a meeting reported in 1928 (ref below). There was general agreement that more experimentation was needed to check Miller’s results. Lorentz recognised that the results, whatever their cause, did not quite tally with either his or Einstein’s versions of special relativity. Einstein was not present at the meeting and felt the results could be dismissed as experimental error (see Shankland ref below).

Name

Year

Arm length (meters)

Fringe Fringe Upper Experimental shift shift Limit on Resolution expected measured Vaether

Michelson

1881

1.2

0.04

0.02

Michelson and Morley

1887

11.0

0.4

< 0.01

Morley and Morley

1902– 1904

32.2

1.13

0.015

Miller

1921

32.0

1.12

0.08

Miller

1923– 1924

32.0

1.12

0.03

Miller (Sunlight)

1924

32.0

1.12

0.014

8 km/s

Tomascheck (Starlight)

1924

8.6

0.3

0.02

Miller

1925– 1926

32.0

1.12

0.088

Mt Wilson)

1926

2.0

0.07

0.002

Illingworth

1927

2.0

0.07

0.0002

Piccard and Stahel (Rigi)

1927

2.8

0.13

0.006

Michelson et al.

1929

25.9

0.9

0.01

Joos

1930

21.0

0.75

0.002

0.0006

1 km/s

In recent times versions of the MM experiment have become commonplace. Lasers and masers amplify light by repeatedly bouncing it back and forth inside a carefully tuned cavity, thereby inducing high-energy atoms in the cavity to give off more light. The result is an effective path length of kilometers. Better yet, the light emitted in one cavity can be used to start the same cascade in another set at right angles, thereby creating an interferometer of extreme accuracy. The first such experiment was led by Charles H. Townes, one of the cocreators of the first maser. Their 1958 experiment put an upper limit on drift, including any possible experimental errors, of only 30 m/s. In 1974 a repeat with accurate lasers in the triangular Trimmer experiment reduced this to 0.025 m/s, and included tests of entrainment by placing one leg in glass. In 1979 the Brillet-Hall experiment put an upper limit of 30 m/s for any one direction, but reduced this to only 0.000001 m/s for a two-direction case (ie, still or partially entrained aether). A year long repeat known as Hils and Hall, published in 1990, reduced this to 2x10-13.

Fallout

This result was rather astounding and not explainable by the then-current theory of wave propagation in a static aether. Several explanations were attempted, among them, that the experiment had a hidden flaw (apparently Michelson’s initial belief), or that the Earth’s gravitational field somehow “dragged” the aether around with it in such a way as locally to eliminate its effect. Miller would have argued that, in most if not all experiments other than his own, there was little possibility of detecting an aether wind since it was almost completely blocked out by the laboratory walls or by the apparatus itself. Be this as it may, the idea of a simple aether, what became known as the First Postulate, had been dealt a serious blow. A number of experiments were carried out to investigate the concept of aether dragging, or entrainment. The most convincing was carried out by Hamar, who placed one arm of the interferometer between two huge lead blocks. If aether were dragged by mass, the blocks would, it was theorised, have been enough to cause a visible effect. Once again, no effect was seen. Walter Ritz’s Emission theory (or ballistic theory), was also consistent with the results of the experiment, not requiring aether, more intuitive and paradox-free. This became known as the Second Postulate. However it also led to several “obvious” optical effects that were not seen in astronomical photographs, notably in observations of binary stars in which the light from the two stars could be measured in an interferometer. The Sagnac experiment placed the MM apparatus on a constantly rotating turntable. In doing so any ballistic theories such as Ritz’s could be tested directly, as the light going one way around the device would have different length to travel than light going the other way (the eyepiece and mirrors would be moving toward/away from the light). In Ritz’s theory there would be no shift, because the net velocity between the light source and detector was zero (they were both mounted on the turntable). However in this case an effect was seen, thereby eliminating any simple ballistic theory. This fringe-shift effect is used today in laser gyroscopes. Another possible solution was found in the Lorentz-FitzGerald contraction hypothesis. In this theory all objects physically contract along the line of motion relative to the aether, so while the light may indeed transit slower on that arm, it also ends up travelling a shorter distance that exactly cancels out the drift. In 1932 the Kennedy-Thorndike experiment modified the Michelson-Morley experiment by making the path lengths of the split beam unequal, with one arm being very long. In this version the two ends of the experiment were at different velocities due to the rotation of the earth, so the contraction would not “work out” to exactly cancel the result. Once again, no effect was seen. Ernst Mach was among the first physicists to suggest that the experiment actually amounted to a disproof of the aether theory. The development of

what became Einstein’s special theory of relativity had the Fitzgerald-Lorentz contraction derived from the invariance postulate, and was also consistent with the apparently null results of most experiments (though not, as was recognised at the 1928 meeting, with Miller’s observed seasonal effects). Today relativity is generally considered the “solution” to the MM null result. The Trouton-Noble experiment is regarded as the electrostatic equivalent of the Michelson-Morley optical experiment, though whether or not it can ever be done with the necessary sensitivity is debatable. On the other hand, the 1908 Trouton-Rankine experiment that spelled the end of the LorentzFitzGerald contraction hypothesis achieved an incredible sensitivity.

Mathematical analysis of the Michelson Morley Experiment The Michelson interferometer splits light into rays that travel along two paths then recombines them. The recombined rays interfere with each other. If the path length changes in one of the arms the interference pattern will shift slightly, moving relative to the cross hairs in the telescope. The Michelson interferometer is arranged as an optical bench on a concrete block that floats on a large pool of mercury. This allows the whole apparatus to be rotated smoothly. If the earth were moving through an aether at the same velocity as it orbits the sun (30 km/sec) then Michelson and Morley calculated that a rotation of the apparatus should cause a shift in the fringe pattern. The basis of this calculation is given below.

Consider the time taken t1 for light to travel along Path 1 in the illustration:

Rearranging terms:

further rearranging:

hence:

Considering Path 2, the light traces out two right angled triangles so:

Rearranging:

So:

It is now easy to calculate the difference (∆t between the times spent by the light in Path 1 and Path 2:

If the apparatus is rotated by 90 degrees the new time difference is:

The interference fringes due to the time difference between the paths will be different after rotation if ∆t and ∆t’ are different.

This difference between the two times can be calculated if the binomial

expansions of

and

are used:

So:

If the period of one vibration of the light is T then the number of fringes (n), that will move past the cross hairs of the telescope when the apparatus is rotated will be:

Inserting the formula for ∆t’ − ∆t:

But cT for a light wave is the wavelength of the light ie: cT = λ so:

If the wavelength of the light is metres then:

and the total path length is 20

So the fringes will shift by 0.4 fringes (ie: 40%) when the apparatus is rotated. However, no fringe shift is observed. The null result of the Michelson-Morley experiment is nowdays explained in terms of the constancy of the speed of light. The assumption that the light would have a velocity of c − v and c + v depending on the direction relative to the hypothetical “aether wind” is false, the light always travels at c between two points in a vacuum and the speed of light is not affected by any “aether wind”. This is because, in {special relativity} the Lorentz transforms induce a {length contraction}. Doing over the above calculations we obtain:

(taking into consideration the length contraction)

It is now easy to recalculate the difference (∆t between the times spent by the light in Path 1 and Path 2:

because

If the apparatus is rotated by 90 degrees the new time difference is:

The interference fringes due to the time difference between the paths will be different after rotation if ∆t and ∆t’ are different.

Coherence length The coherence length of light rays from a source that has wavelengths that differ by ∆λ is:

If path lengths differ by more than this amount then interference fringes will not be observed. White light has a wide range of wavelengths and

interferometers using white light must have paths that are equal to within a small fraction of a millimetre for interference to occur. This means that the ideal light source for a Michelson Interferometer should be monochromatic and the arms should be as near as possible equal in length. The calculation of the coherence length is based on the fact that interference fringes become unclear when light rays are about 60 degrees (about 1 radian or one sixth of a wavelength ( two beams are:

)) out of phase. This means that when

metres out of step they will no longer give a well defined interference pattern. Suppose a light beam contains two wavelengths of light, λ and λ + ∆λ, then in:

cycles they will be

out of phase.

The distance required for the two different wavelengths of light to be this much out of phase is the coherence length. Coherence length = number of cycles x length of each cycle so:

coherence length =

.

Lorentz-Fitzgerald Contraction Hypothesis After the first Michelson-Morley experiments in 1881 there were several attempts to explain the null result. The most obvious point of attack is to propose that the Path that is parallel to the direction of motion is contracted by in which case ∆t and ∆t’ would be identical and no fringe shift would occur. This possibility was proposed in 1892 by Fitzgerald. Lorentz produced an “electron theory of matter” that would account for such a contraction. Students sometimes make the mistake of assuming that the LorentzFitzgerald contraction is equivalent to the Lorentz transformations. However, in absence of any treatment of time dilation effect the Lorentz-Fitgerald

explanation would result in a fringe shift if the apparatus is moved between two different velocities. The rotation of the earth allows this effect to be tested as the earth orbits the sun. Kennedy and Thorndike (1932) performed the Michelson-Morley experiment with a highly sensitive apparatus that could detect any effect due to the rotation of the earth; they found no effect. They concluded that both time dilation and Lorentz-Fitzgerald Contraction take place, thus confirming relativity theory. The fringe shifts due to velocity changes if only the Lorentz-Fitzgerald contraction applied would be: . Notice how the sensitivity of the experiment is dependent on the difference in path length Lf − Lm and hence a long coherence length is required.

Mathematical Approach Vectors Physical effects involve things acting on other things to produce a change of position, tension etc. These effects usually depend upon the strength, angle of contact, separation etc of the interacting things rather than on any absolute reference frame so it is useful to describe the rules that govern the interactions in terms of the relative positions and lengths of the interacting things rather than in terms of any fixed viewpoint or coordinate system. Vectors were introduced in physics to allow such relative descriptions. The use of vectors in elementary physics often avoids any real understanding of what they are. They are a new concept, as unique as numbers themselves, which have been related to the rest of mathematics and geometry by a series of formulae such as linear combinations, scalar products etc. Vectors are defined as “directed line segments” which means they are lines drawn in a particular direction. The introduction of time as a geometric entity means that this definition of a vector is rather archaic, a better definition might be that a vector is information arranged as a continuous succession of points in space and time. Vectors have length and direction, the direction being from earlier to later. Vectors are represented by lines terminated with arrow symbols to show the direction. A point that moves from the left to the right for about three centimetres can be represented as:

If a vector is represented within a coordinate system it has components along each of the axes of the system. These components do not normally start at the origin of the coordinate system.

The vector represented by the bold arrow has components a, b and c which are lengths on the coordinate axes. If the vector starts at the origin the

components become simply the coordinates of the end point of the vector and the vector is known as the position vector of the end point.

Addition of Vectors If two vectors are connected so that the end point of one is the start of the next the sum of the two vectors is defined as a third vector drawn from the start of the first to the end of the second:

c is the sum of a and b: c=a+b If a components of a are a, b, c and the components of b are d, e, f then the components of the sum of the two vectors are (a+d), (b+e) and (c+f). In other words, when vectors are added it is the components that add numerically rather than the lengths of the vectors themselves. Rules of Vector Addition 1. Commutativity a + b = b + a 2. Associativity (a + b) + c = a + (b + c) If the zero vector (which has no length) is labelled as 0 3. a + (-a) = 0 4. a + 0 = a

Rules of Vector Multiplication by a Scalar The discussion of components and vector addition shows that if vector a has components a,b,c then qa has components qa, qb, qc. The meaning of vector multiplication is shown below:

The bottom vector c is added three times which is equivalent to multiplying it by 3.

1. Distributive laws q(a + b) = qa + qb and (q + p)a = qa + pa 2. Associativity q(pa) = qpa Also 1 a = a If the rules of vector addition and multiplication by a scalar apply to a set of elements they are said to define a vector space.

Linear Combinations and Linear Dependence An element of the form:

is called a linear combination of the vectors. The set of vectors multiplied by scalars in a linear combination is called the span of the vectors. The word span is used because the scalars (q) can have any value - which means that any point in the subset of the vector space defined by the span can contain a vector derived from it. Suppose there were a set of vectors (a1,a2,....,am) , if it is possible to express one of these vectors in terms of the others, using any linear combination, then the set is said to be linearly dependent. If it is not possible to express any one of the vectors in terms of the others, using any linear combination, it is said to be linearly independent. In other words, if there are values of the scalars such that: (1). the set is said to be linearly dependent. There is a way of determining linear dependence. From (1) it can be seen that if q1 is set to minus one then:

So in general, if a linear combination can be written that sums to a zero vector then the set of vectors (

are not linearly independent.

If two vectors are linearly dependent then they lie along the same line (wherever a and b lie on the line, scalars can be found to produce a linear combination which is a zero vector). If three vectors are linearly dependent they lie on the same line or on a plane (collinear or coplanar).

Dimension If n+1 vectors in a vector space are linearly dependent then n vectors are linearly independent and the space is said to have a dimension of n. The set of n vectors is said to be the basis of the vector space.

Scalar Product Also known as the ‘dot product’ or ‘inner product’. The scalar product is a way of removing the problem of angular measures from the relationship between vectors and, as Weyl put it, a way of comparing the lengths of vectors that are arbitrarily inclined to each other. Consider two vectors with a common origin:

The projection of

Where

on

is:

is the length of

.

The scalar product is defined as: (2) Notice that cos q is zero if and are perpendicular. This means that if the scalar product is zero the vectors composing it are orthogonal (perpendicular to each other). (2) also allows cos q to be defined as:

The definition of the scalar product also allows a definition of the length of a vector in terms of the concept of a vector itself. The scalar product of a vector with itself is:

cos 0 (the cosine of zero) is one so:

which is our first direct relationship between vectors and scalars. This can be expressed as: (3) where a is the length of

.

Properties: 1. Linearity 2. symmetry 3. Positive definiteness

is greater than or equal to 0

4. Distributivity for vector addition 5. Schwarz inequality 6. Parallelogram equality From the point of view of vector physics the most important property of the scalar product is the expression of the scalar product in terms of coordinates. 7. This gives us the length of a vector in terms of coordinates (Pythagoras’ theorem) from: 8. The derivation of 7 is:

where

are unit vectors along the coordinate axes. From (4)

but so:

etc. are all zero because the vectors are orthogonal, also and length).

are all one (these are unit vectors defined to be 1 unit in

Using these results:

Matrices Matrices are sets of numbers arranged in a rectangular array. They are especially important in linear algebra because they can be used to represent the elements of linear equations. 11a + 2b = c 5a + 7b = d The constants in the equation above can be represented as a matrix:

The elements of matrices are usually denoted symbolically using lower case letters:

Matrices are said to be equal if all of the corresponding elements are equal. Eg: if aij = bij Then

Matrix Addition

Matrices are added by adding the individual elements of one matrix to the corresponding elements of the other matrix. cij = aij + bij or Matrix addition has the following properties: 1. Commutativity 2. Associativity and 3. 4. From matrix addition it can be seen that the product of a matrix number p is simply individually by p.

and a

where every element of the matrix is multiplied

Transpose of a Matrix A matrix is transposed when the rows and columns are interchanged:

Notice that the principal diagonal elements stay the same after transposition. A matrix is symmetric if it is equal to its transpose eg: akj = ajk. It is skew symmetric if eg: akj = − ajk. The principal diagonal of a skew symmetric matrix is composed of elements that are zero.

Other Types of Matrix Diagonal matrix: all elements above and below the principal diagonal are zero.

Unit matrix: denoted by I, is a diagonal matrix where all elements of the principal diagonal are 1.

Matrix Multiplication Matrix multiplication is defined in terms of the problem of determining the coefficients in linear transformations. Consider a set of linear transformations between 2 coordinate systems that share a common origin and are related to each other by a rotation of the coordinate axes. Two Coordinate Systems Rotated Relative to Each Other If there are 3 coordinate systems, x, y, and z these can be transformed from one to another: x1 = a11y1 + a12y2 x2 = a21y1 + a22y2

y1 = b11z1 + b12z2 y2 = b21z1 + b22z2

x1 = c11z1 + c12z2

x2 = c21z1 + c22z2

By substitution: x1 = a11(b11z1 + b12z2) + a12(b21z1 + b22z2) x2 = a21(b11z1 + b12z2) + a22(b21z1 + b22z2) x1 = (a11b11 + a12(b21)z1 + (a11b12 + a12b22)z2 x2 = (a21b11 + a22(b21)z1 + (a21b12 + a22b22)z2 Therefore: c11 = (a11b11 + a12(b21) c12 = (a11b12 + a12b22) c21 = (a21b11 + a22b21) c22 = (a21b12 + a22b22) The coefficient matrices are:

From the linear transformation the product of A and B is defined as:

In the discussion of scalar products it was shown that, for a plane the scalar product is calculated as: coordinates of the vectors a and b.

where a and b are the

Now mathematicians define the rows and columns of a matrix as vectors:

A Column vector is And a Row vector

Matrices can be described as vectors eg:

and

Matrix multiplication is then defined as the scalar products of the vectors so that:

From the definition of the scalar product

etc.

In the general case:

This is described as the multiplication of rows into columns (eg: row vectors into column vectors). The first matrix must have the same number of columns as there are rows in the second matrix or the multiplication is undefined.

After matrix multiplication the product matrix has the same number of rows as the first matrix and columns as the second matrix:

times

has 2 rows and 1 column

ie: first row is 1 * 2 + 3 * 3 + 4 * 7 = 39 and second row is 6 * 2 + 3 * 3 + 2 * 7 = 35

times

has 2 rows and 3

columns Notice that cannot be determined because the number of columns in the first matrix must equal the number of rows in the second matrix to perform matrix multiplication. Properties of Matrix Multiplication 1. Not commutative 2. Associative

3. Distributative for matrix addition

matrix multiplication is not commutative so separate case. 4. The cancellation law is not always true: does not mean

or

is a

There is a case where matrix multiplication is commutative. This involves the scalar matrix where the values of the principle diagonal are all equal. Eg:

In this case

. If the scalar matrix is the unit matrix: .

Linear Transformations A simple linear transformation such as: x1 = a11y1 + a12y2 x2 = a21y1 + a22y2 can be expressed as:

eg:

and y1 = b11z1 + b12z2 y2 = b21z1 + b22z2

as: Using the associative law:

and so:

as before.

Indicial Notation Consider a simple rotation of coordinates:

xµ is defined as x1 , x2 xν is defined as

,

The scalar product can be written as:

Where:

and is called the metric tensor for this 2D space.

Now, g11 = 1, g12 = 0, g21 = 0, g22 = 1 so:

If there is no rotation of coordinates the scalar product is:

Which is Pythagoras’ theorem.

The Summation Convention Indexes that appear as both subscripts and superscripts are summed over.

by promoting n to a superscript it is taken out of the summation ie:.

where ν = 1

Matrix Multiplication in Indicial Notation Consider: Columns times rows:

times

Matrix product

=

Where i = 1, 2 j = 1, 2

There being no summation the indexes are both subscripts.

Rows times columns:

times

=

Matrix product Where δij is known as Kronecker delta and has the value 0 when when i = j. It is the indicial equivalent of the unit matrix:

and 1

There being summation one value of i is a subscript and the other a superscript. A matrix in general can be specified by any of: , Mij , , Mij depending on which subscript or superscript is being summed over.

Vectors in Indicial Notation A vector can be expressed as a sum of basis vectors.

In indicial notation this is: x = aiei

Linear Transformations in indicial notation Consider where coordinate matrices.

is

a

coefficient

matrix

and

and

are

In indicial notation this is:

which becomes:

The Scalar Product in indicial notation In indicial notation the scalar product is:

Analysis of curved transformations

surfaces

and

It became apparent at the start of the nineteenth century that issues such as Euclid’s parallel postulate required the development of a new type of geometry that could deal with curved surfaces and real and imaginary planes. At the foundation of this approach is Gauss’s analysis of curved surfaces which allows us to work with a variety of coordinate systems and displacements on any type of surface.

Elementary geometric analysis is useful as an introduction to Special Relativity because it suggests the physical meaning of the coefficients that appear in coordinate transformations. Suppose there is a line on a surface. The length of this line can be expressed in terms of a coordinate system. A short length of line ∆s in a two dimensional space may be expressed in terms of Pythagoras’ theorem as: ∆s2 = ∆x2 + ∆y2 Suppose there is another coordinate system on the surface with two axes: x1, x2, how can the length of the line be expressed in terms of these coordinates? Gauss tackled this problem and his analysis is quite straightforward for two coordinate axes: Figure 1:

It is possible to use elementary differential geometry to describe displacements along the plane in terms of displacements on the curved surfaces:

The displacement of a short line is then assumed to be given by a formula, called a metric, such as Pythagoras’ theorem

∆S2 = ∆Y2 + ∆Z2 The values of ∆Y and ∆Z can then be substituted into this metric:

Which, when expanded, gives the following: ∆S2 =

This can be represented using summation notation:

Or, using indicial notation: ∆S2 = gik∆xi∆xk Where:

If the coordinates are not merged then ∆s is dependent on both sets of coordinates. In matrix notation:

becomes:

times

times

Where a, b, c, d stand for the values of gik. Therefore:

times Which is:

So:

∆s2 is a bilinear form that depends on both ∆x1 and ∆x2. It can be written in matrix notation as:

Where A is the matrix containing the values in gik. This is a special case of the bilinear form known as the quadratic form because the same matrix ( ) appears twice; in the generalised bilinear form matrices and are different). If the surface is a Euclidean plane then the values of gik are:

Which become:

So the matrix A is the unit matrix I and:

(the

and:

Which recovers Pythagoras’ theorem yet again. If the surface is derived from some other metric such as ∆s2 = − ∆Y2 + ∆Z2 then the values of gik are:

Which becomes:

Which

allows

the

original

metric

to

be

recovered

ie:

. It is interesting to compare the geometrical analysis with the transformation based on matrix algebra that was derived in the section on indicial notation above:

Now,

ie: g11 = 1, g12 = 0, g21 = 0, g22 = 1 so:

If there is no rotation of coordinates the scalar product is:

Which recovers Pythagoras’ theorem. However, the reader may have noticed that Pythagoras’ theorem had been assumed from the outset in the derivation of the scalar product (see above). The geometrical analysis shows that if a metric is assumed and the conditions that allow differential geometry are present then it is possible to derive one set of coordinates from another. This analysis can also be performed using matrix algebra with the same assumptions. The example above used a simple two dimensional Pythagorean metric, some other metric such as the metric of a 4D Minkowskian space: ∆S2 = − ∆T2 + ∆X2 + ∆Y2 + ∆Z2 could be used instead of Pythagoras’ theorem.

Mathematical Approach2 Vectors Physical effects involve things acting on other things to produce a change of position, tension etc. These effects usually depend upon the strength, angle of contact, separation etc of the interacting things rather than on any absolute reference frame so it is useful to describe the rules that govern the interactions in terms of the relative positions and lengths of the interacting things rather than in terms of any fixed viewpoint or coordinate system. Vectors were introduced in physics to allow such relative descriptions. The use of vectors in elementary physics often avoids any real understanding of what they are. They are a new concept, as unique as numbers themselves, which have been related to the rest of mathematics and geometry by a series of formulae such as linear combinations, scalar products etc. Vectors are defined as “directed line segments” which means they are lines drawn in a particular direction. The introduction of time as a geometric entity means that this definition of a vector is rather archaic, a better definition might be that a vector is information arranged as a continuous succession of points in space and time. Vectors have length and direction, the direction being from earlier to later. Vectors are represented by lines terminated with arrow symbols to show the direction. A point that moves from the left to the right for about three centimetres can be represented as:

If a vector is represented within a coordinate system it has components along each of the axes of the system. These components do not normally start at the origin of the coordinate system.

The vector represented by the bold arrow has components a, b and c which are lengths on the coordinate axes. If the vector starts at the origin the

components become simply the coordinates of the end point of the vector and the vector is known as the position vector of the end point.

Addition of Vectors If two vectors are connected so that the end point of one is the start of the next the sum of the two vectors is defined as a third vector drawn from the start of the first to the end of the second:

c is the sum of a and b: c=a+b If a components of a are a, b, c and the components of b are d, e, f then the components of the sum of the two vectors are (a+d), (b+e) and (c+f). In other words, when vectors are added it is the components that add numerically rather than the lengths of the vectors themselves. Rules of Vector Addition 1. Commutativity a + b = b + a 2. Associativity (a + b) + c = a + (b + c) If the zero vector (which has no length) is labelled as 0 3. a + (-a) = 0 4. a + 0 = a

Rules of Vector Multiplication by a Scalar The discussion of components and vector addition shows that if vector a has components a,b,c then qa has components qa, qb, qc. The meaning of vector multiplication is shown below:

The bottom vector c is added three times which is equivalent to multiplying it by 3.

1. Distributive laws q(a + b) = qa + qb and (q + p)a = qa + pa 2. Associativity q(pa) = qpa Also 1 a = a If the rules of vector addition and multiplication by a scalar apply to a set of elements they are said to define a vector space.

Linear Combinations and Linear Dependence An element of the form:

is called a linear combination of the vectors. The set of vectors multiplied by scalars in a linear combination is called the span of the vectors. The word span is used because the scalars (q) can have any value - which means that any point in the subset of the vector space defined by the span can contain a vector derived from it. Suppose there were a set of vectors (a1,a2,....,am) , if it is possible to express one of these vectors in terms of the others, using any linear combination, then the set is said to be linearly dependent. If it is not possible to express any one of the vectors in terms of the others, using any linear combination, it is said to be linearly independent. In other words, if there are values of the scalars such that: (1). the set is said to be linearly dependent. There is a way of determining linear dependence. From (1) it can be seen that if q1 is set to minus one then:

So in general, if a linear combination can be written that sums to a zero vector then the set of vectors (

are not linearly independent.

If two vectors are linearly dependent then they lie along the same line (wherever a and b lie on the line, scalars can be found to produce a linear combination which is a zero vector). If three vectors are linearly dependent they lie on the same line or on a plane (collinear or coplanar).

Dimension If n+1 vectors in a vector space are linearly dependent then n vectors are linearly independent and the space is said to have a dimension of n. The set of n vectors is said to be the basis of the vector space.

Scalar Product Also known as the ‘dot product’ or ‘inner product’. The scalar product is a way of removing the problem of angular measures from the relationship between vectors and, as Weyl put it, a way of comparing the lengths of vectors that are arbitrarily inclined to each other. Consider two vectors with a common origin:

The projection of

Where

on

is:

is the length of

.

The scalar product is defined as: (2) Notice that cos q is zero if and are perpendicular. This means that if the scalar product is zero the vectors composing it are orthogonal (perpendicular to each other). (2) also allows cos q to be defined as:

The definition of the scalar product also allows a definition of the length of a vector in terms of the concept of a vector itself. The scalar product of a vector with itself is:

cos 0 (the cosine of zero) is one so:

which is our first direct relationship between vectors and scalars. This can be expressed as: (3) where a is the length of

.

Properties: 1. Linearity 2. symmetry 3. Positive definiteness

is greater than or equal to 0

4. Distributivity for vector addition 5. Schwarz inequality 6. Parallelogram equality From the point of view of vector physics the most important property of the scalar product is the expression of the scalar product in terms of coordinates. 7. This gives us the length of a vector in terms of coordinates (Pythagoras’ theorem) from: 8. The derivation of 7 is:

where

are unit vectors along the coordinate axes. From (4)

but so:

etc. are all zero because the vectors are orthogonal, also and length).

are all one (these are unit vectors defined to be 1 unit in

Using these results:

Matrices Matrices are sets of numbers arranged in a rectangular array. They are especially important in linear algebra because they can be used to represent the elements of linear equations. 11a + 2b = c 5a + 7b = d The constants in the equation above can be represented as a matrix:

The elements of matrices are usually denoted symbolically using lower case letters:

Matrices are said to be equal if all of the corresponding elements are equal. Eg: if aij = bij Then

Matrix Addition

Matrices are added by adding the individual elements of one matrix to the corresponding elements of the other matrix. cij = aij + bij or Matrix addition has the following properties: 1. Commutativity 2. Associativity and 3. 4. From matrix addition it can be seen that the product of a matrix number p is simply individually by p.

and a

where every element of the matrix is multiplied

Transpose of a Matrix A matrix is transposed when the rows and columns are interchanged:

Notice that the principal diagonal elements stay the same after transposition. A matrix is symmetric if it is equal to its transpose eg: akj = ajk. It is skew symmetric if eg: akj = − ajk. The principal diagonal of a skew symmetric matrix is composed of elements that are zero.

Other Types of Matrix Diagonal matrix: all elements above and below the principal diagonal are zero.

Unit matrix: denoted by I, is a diagonal matrix where all elements of the principal diagonal are 1.

Matrix Multiplication Matrix multiplication is defined in terms of the problem of determining the coefficients in linear transformations. Consider a set of linear transformations between 2 coordinate systems that share a common origin and are related to each other by a rotation of the coordinate axes. Two Coordinate Systems Rotated Relative to Each Other If there are 3 coordinate systems, x, y, and z these can be transformed from one to another: x1 = a11y1 + a12y2 x2 = a21y1 + a22y2

y1 = b11z1 + b12z2 y2 = b21z1 + b22z2

x1 = c11z1 + c12z2

x2 = c21z1 + c22z2

By substitution: x1 = a11(b11z1 + b12z2) + a12(b21z1 + b22z2) x2 = a21(b11z1 + b12z2) + a22(b21z1 + b22z2) x1 = (a11b11 + a12(b21)z1 + (a11b12 + a12b22)z2 x2 = (a21b11 + a22(b21)z1 + (a21b12 + a22b22)z2 Therefore: c11 = (a11b11 + a12(b21) c12 = (a11b12 + a12b22) c21 = (a21b11 + a22b21) c22 = (a21b12 + a22b22) The coefficient matrices are:

From the linear transformation the product of A and B is defined as:

In the discussion of scalar products it was shown that, for a plane the scalar product is calculated as: coordinates of the vectors a and b.

where a and b are the

Now mathematicians define the rows and columns of a matrix as vectors:

A Column vector is And a Row vector

Matrices can be described as vectors eg:

and

Matrix multiplication is then defined as the scalar products of the vectors so that:

From the definition of the scalar product

etc.

In the general case:

This is described as the multiplication of rows into columns (eg: row vectors into column vectors). The first matrix must have the same number of

columns as there are rows in the second matrix or the multiplication is undefined. After matrix multiplication the product matrix has the same number of rows as the first matrix and columns as the second matrix:

times

has 2 rows and 1 column

ie: first row is 1 * 2 + 3 * 3 + 4 * 7 = 39 and second row is 6 * 2 + 3 * 3 + 2 * 7 = 35

times

has 2 rows and 3

columns Notice that cannot be determined because the number of columns in the first matrix must equal the number of rows in the second matrix to perform matrix multiplication. Properties of Matrix Multiplication 1. Not commutative 2. Associative

3. Distributative for matrix addition

matrix multiplication is not commutative so separate case. 4. The cancellation law is not always true: does not mean

or

is a

There is a case where matrix multiplication is commutative. This involves the scalar matrix where the values of the principle diagonal are all equal. Eg:

In this case

. If the scalar matrix is the unit matrix: .

Linear Transformations A simple linear transformation such as: x1 = a11y1 + a12y2 x2 = a21y1 + a22y2 can be expressed as:

eg:

and y1 = b11z1 + b12z2 y2 = b21z1 + b22z2

as: Using the associative law:

and so:

as before.

Indicial Notation Consider a simple rotation of coordinates:

xµ is defined as x1 , x2 xν is defined as

,

The scalar product can be written as:

Where:

and is called the metric tensor for this 2D space.

Now, g11 = 1, g12 = 0, g21 = 0, g22 = 1 so:

If there is no rotation of coordinates the scalar product is:

Which is Pythagoras’ theorem.

The Summation Convention Indexes that appear as both subscripts and superscripts are summed over.

by promoting n to a superscript it is taken out of the summation ie:.

where ν = 1

Matrix Multiplication in Indicial Notation Consider: Columns times rows:

times

Matrix product

=

Where i = 1, 2 j = 1, 2

There being no summation the indexes are both subscripts.

Rows times columns:

times

=

Matrix product Where δij is known as Kronecker delta and has the value 0 when when i = j. It is the indicial equivalent of the unit matrix:

and 1

There being summation one value of i is a subscript and the other a superscript. A matrix in general can be specified by any of: , Mij , , Mij depending on which subscript or superscript is being summed over.

Vectors in Indicial Notation A vector can be expressed as a sum of basis vectors.

In indicial notation this is: x = aiei

Linear Transformations in indicial notation Consider where coordinate matrices.

is

a

coefficient

matrix

and

and

are

In indicial notation this is:

which becomes:

The Scalar Product in indicial notation In indicial notation the scalar product is:

Analysis of curved transformations

surfaces

and

It became apparent at the start of the nineteenth century that issues such as Euclid’s parallel postulate required the development of a new type of geometry that could deal with curved surfaces and real and imaginary planes. At the foundation of this approach is Gauss’s analysis of curved surfaces which allows us to work with a variety of coordinate systems and displacements on any type of surface.

Elementary geometric analysis is useful as an introduction to Special Relativity because it suggests the physical meaning of the coefficients that appear in coordinate transformations. Suppose there is a line on a surface. The length of this line can be expressed in terms of a coordinate system. A short length of line ∆s in a two dimensional space may be expressed in terms of Pythagoras’ theorem as: ∆s2 = ∆x2 + ∆y2 Suppose there is another coordinate system on the surface with two axes: x1, x2, how can the length of the line be expressed in terms of these coordinates? Gauss tackled this problem and his analysis is quite straightforward for two coordinate axes: Figure 1:

It is possible to use elementary differential geometry to describe displacements along the plane in terms of displacements on the curved surfaces:

The displacement of a short line is then assumed to be given by a formula, called a metric, such as Pythagoras’ theorem

∆S2 = ∆Y2 + ∆Z2 The values of ∆Y and ∆Z can then be substituted into this metric:

Which, when expanded, gives the following: ∆S2 =

This can be represented using summation notation:

Or, using indicial notation: ∆S2 = gik∆xi∆xk Where:

If the coordinates are not merged then ∆s is dependent on both sets of coordinates. In matrix notation:

becomes:

times

times

Where a, b, c, d stand for the values of gik. Therefore:

times Which is:

So:

∆s2 is a bilinear form that depends on both ∆x1 and ∆x2. It can be written in matrix notation as:

Where A is the matrix containing the values in gik. This is a special case of the bilinear form known as the quadratic form because the same matrix ( ) appears twice; in the generalised bilinear form matrices and are different). If the surface is a Euclidean plane then the values of gik are:

Which become:

So the matrix A is the unit matrix I and:

(the

and:

Which recovers Pythagoras’ theorem yet again. If the surface is derived from some other metric such as ∆s2 = − ∆Y2 + ∆Z2 then the values of gik are:

Which becomes:

Which

allows

the

original

metric

to

be

recovered

ie:

. It is interesting to compare the geometrical analysis with the transformation based on matrix algebra that was derived in the section on indicial notation above:

Now,

ie: g11 = 1, g12 = 0, g21 = 0, g22 = 1 so:

If there is no rotation of coordinates the scalar product is:

Which recovers Pythagoras’ theorem. However, the reader may have noticed that Pythagoras’ theorem had been assumed from the outset in the derivation of the scalar product (see above). The geometrical analysis shows that if a metric is assumed and the conditions that allow differential geometry are present then it is possible to derive one set of coordinates from another. This analysis can also be performed using matrix algebra with the same assumptions. The example above used a simple two dimensional Pythagorean metric, some other metric such as the metric of a 4D Minkowskian space: ∆S2 = − ∆T2 + ∆X2 + ∆Y2 + ∆Z2 could be used instead of Pythagoras’ theorem.

Mathematical Transformations Mathematical approach special relativity

to

Introduction The teaching of Special Relativity on undergraduate physics courses involves a considerable mathematical background knowledge. Particularly important are the manipulation of vectors and matrices and an elementary knowledge of curvature. The background mathematics is given at the end of this section and can be referenced by those who are unfamiliar with these techniques.

The Lorentz transformation The Lorentz transformation deals with the problem of observers who are moving relative to each other. How are the coordinates of an event recorded by one observer related to the coordinates of the event recorded by the other observer? The standard configuration used in the calculation of the Lorentz transformation is shown below:

There are several ways of deriving the Lorentz transformations. The usual method is to work from Einstein’s postulates (that the laws of physics are the

same between all inertial reference frames and the speed of light is constant) whilst adding assumptions about isotropy, linearity and homogeneity. The second is to work from the assumption of a four dimensional Minkowskian metric. In mathematics transformations are frequently symbolised with the “maps to” symbol:

The linearity and homogeneity of spacetime Consider a clock moving freely, according to Newton’s first law, that objects continue in a state of uniform motion unless acted upon by a force, the velocity of the clock in any given direction (dxi / dt) is a constant. If the clock is a real clock with readings given by τ then the relationship between these readings and the elapsed time anywhere in an inertial frame of reference, dt / dτ, will be a constant. If the clock were to tick at an uneven rate compared with other clocks then the universe would not be homogenous in time - at some times the clock would appear to accelerate. This would also mean that Newton’s first law would be broken and the universe would not be homogenous in space. If dxi / dt and dt / dτ are constant then dxµ(µ = 1,2,3,4) is also constant. This means that the clock is not accelerating ie: d2xµ / dτ2 = 0. Linearity is demonstrated by the way that the length of things does not depend on position or relative position; for instance, if x’ = ax2 the distance between two points would depend upon the position of the observer whereas if the relationship is linear (x’ = ax) separations are independent of position. The linearity and homogeneity assumptions mean that the coordinates of objects in the S’ inertial frame are related to those in the S inertial frame by:

This formula is known as a Poincaré transformation. It can be expressed in indicial notation as:

If the origins of the frames coincide then Bν can be assumed to be zero and the equation:

Those who are unfamiliar with the notation should note that the symbols x1 etc. mean x1 = x,x2 = y,x3 = z,x4 = t so the equation above is shorthand for: x’ = a11x + a12y + a13z + a14t y’ = a21x + a22y + a23z + a24t z’ = a31x + a32y + a33z + a34t t’ = a41x + a42y + a43z + a44t In matrix notation the set of equations can be written as:

The standard configuration (see diagram above) has several properties, for instance:

The spatial origin of both observer’s coordinate systems lies on the line of motion so the x axes can be chosen to be parallel. The point given by x = vt is the same as x’ = 0. The origins of both coordinate systems can coincide so that clocks can be synchronised when they are next to each other. The coordinate planes ,y, y’ and z,z’, can be arranged to be orthogonal (at right angles) to the direction of motion. Isotropy means that coordinate planes that are orthogonal at y=0 and z=0 in one frame are orthogonal at y’=0 and z’=0 in the other frame. According to the relativity principle any transformations between the same two inertial frames of reference must be the same. This is known as the reciprocity theorem.

The Lorentz transformation From the linearity assumption and given that at the origin y = 0 = y’ so there is no constant offset then y’ = Ky and y = Ky’, therefore K=1. So: y’ = y and, by the same reasoning: z’ = z

Now, considering the x coordinate of the event, the z and y axes can be assumed to be 0 (ie: an arbitrary shift of the coordinates to allow the event to lie on the x axes). If this is done then the linearity consideration and the fact that x = vt and x’ = 0 are the same point gives: (1) x’ = γ(x − vt) where γ is a constant. According to the reciprocity theorem we also have: (2) x = γ(x’ + vt’) Einstein’s assumption that the speed of light is a constant can now be introduced so that x = ct and also x’ = ct’. So: ct’ = γt(c − v) and ct = γt’(c + v) So: c2tt’ = γ2tt’(c2 − v2) and

Therefore the Lorentz transformation equations are: t’ = γ(t − vx / c2) x’ = γ(x − vt) y’ = y z’ = z The transformation for the time coordinate can derived from the transformation for the x coordinate assuming x = ct and x’ = ct’ or directly from equations (1) and (2) with a similar substitution for x = ct. The coefficients of the Lorentz transformation can be represented in matrix format:

A coordinate transformation of this type, that is due to motion, is known as a boost.

The Lorentz transformation equations can be used to show that: c2dt’2 − dx’2 − dy’2 − dz’2 = c2dt2 − dx2 − dy2 − dz2 Although whether the assumptions of linearity, isotropy and homogeneity in the derivation of the Lorentz transformation actually assumed this identity from the outset is a mute point. Given that: c2dt’2 − dx’2 − dy’2 − dz’2 also equals c2dt”2 − dx”2 − dy”2 − dz”2 and a continuous range of other transformations it is clear that:

The quantity ∆s is known as the spacetime interval and the quantity known as the squared displacement.

is

A given squared displacement is constant for all observers, no matter how fast they are travelling, it is said to be invariant. The equation: ds2 = c2dt2 − dx2 − dy2 − dz2 is known as the metric of spacetime.

Another look at the Lorentz transformation

We have seen how length and duration both look different to a moving observer:

• •

lengths are contracted by a factor γ, when measured at the same time. durations are contracted by a factor γ, when measured in the same place.

We can extend these results to allow for measurements taking place at different times and places. Lets consider two observers, O and O’ such that O’ is moving at velocity v along the x-axis with respect to O. We’ll use primed variables for all the measurements O’ makes. We can assume for now both observers have the same origin and x-axis because we already know how to allow for observers being relatively rotated and displaced. We can put these complications back in later. Now any length or duration can be written as the difference between two coordinates, for the two ends of the body or the start and end of the event, so it is sufficient to know how to change coordinates from one frame to the other. We know how to do this in classical physics,

we need to extend this to relativity. Notice that the in classical physics the relationship is linear; the graphs of these equations are straight lines. This makes the maths much simpler, so we will try to find a linear relationship between the coordinates for relativity, i.e equations of the general form

where m, n, p and q are all independent of the coordinates. To begin with know that

• •

when t=0, x′ = γx (Lorentz contraction) when x=0, t′ = γt (Time dilation)

and that O’ is travelling at velocity v. They measure their position to be at x′=0, but O measures it to be at vt so we must have

x′=0 when x-vt=0 The only relationship between x and x′ that satisfies these criteria is

Both observers must measure the same speed for light,

or, substituting and rearranging,

The only linear relationship between t and t′ that satisfies these criteria is

So the primed and unprimed coordinates are related by

These equations are called the Lorentz transform. They look simpler if we write them in terms of ct rather than t

Written this way they look much like the equations describing a rotation in three dimensions. In fact, once we allow for the different Pythagorean theorem, they are exactly like the equations for rotation.

•

If observers are moving relative to each other, their coordinate systems are rotated in the (x,ct) plane.

The derivation of the constancy of the speed of light from the first postulate alone The following text was taken from a “Wikiversity” lecture.

The lorentz transformation can be deduced from the first postulate alone with no additional assumptions. The Lorentz transformation describes the way a vector in spacetime as seen by an observer O1 changes when it is seen by an observer O2 in a different inertial system. The postulate of special relativity states that the laws of physics are independent of the velocity of an observer. This is equivalent to the requirement that the mathematical formulation of the laws of physics must be invariant if the coordinates of space and time are altered using a lorentz transformation. This is the reason why the Lorentz transformation is a central concept in special relativity. Four-vectors are vectors that, subjected to the Lorentz transformation, behave like vectors from spacetime. The standard approach to find relativistic laws to the corresponding non-relativistic ones is to find fourvectors to replace space vectors. Surprisingly often this simple approach is successful. Here, for simplicity, the space coordinates are choosen so that the x-axis points in the direction in which O2 is moving with respect to O1 and the y and z coordinates are not considered. Here is a way to determine a matrix L(v) that transforms coordinates from O1 to O2 where v is the velocity of O2 seen from O1.

Most generally, this matrix looks like

to

via

and transforms

.

The first requirement on this matrix is that O2 will be at rest (in space) within its own coordinate system.

Therfore

This gives us

for any time t.

and

.

The backward transformation from O2 to O1 is described by the inverse

matrix (where is the velocity of O1 seen from O2) has to leave O1 at rest in its own coordinates. Assuming v’ = - v (being strict this would have to be proved) we have

for

any

time

resulting

in

. The coefficients are functions of v. For all v the transformation is invertible, so

.

For v=0 L(v) is the identity matrix, so b(0)=0. This allows to rewrite with a yet unknown function

Now

.

.

Now for every v0,v1 there exists a velocity v2 such that L(v0)L(v1) = L(v2). Using what we have proved up to now gives

. Comparing

the

diagonal

components,

we

see

immediately

for arbitrary values of v0,v1, and so Using

this

we

can

rewrite

the

that .

equation

for

the

and by comparing each of the components

at

indeces

(1,1)

and

(2,1)

respectively.

Substituing

a(v2)

yields

from which we have

.

Reinserting

this

into

the

first

equation

we

have

. In case v: = v0 = - v1 this simplifies to

as

is the identity matrix.

Let

be an even function and ψ(x) = - ψ( - x) an odd function

such that This

gives

. us

.

Taking

logarithm and simplifying yields

we

from which

get

with

any

odd

. The negative sign is ruled out by

Substituting

the

function

.

in equation (1) verifies the solution but leaves us with .

To prove we flip the direction of the x-axis in the base system as well as in the transformed system. The choice of the direction of the axis must not lead to different results. In the flipped coordinates the Lorentz transformation takes the form

for v’ = - v. On the other hand we transformation on L(v):

can

compute

L(v’)

by

using

a

coordinate

from which we get

and hence this gives

. Together with

.

Looking at the eigenvectors of L(v) we see they are v.

independent of

This is equivalent to a transformation invariant velocity which experimentation shows to be equal to the speed of light. Altogether we have:

Note that for we would have the Galileian case with no finite invariant velocity. Behold that if we did the same deduction not in the spacetime plane for (t, x) but in the space plane (x, y) with slope m=y/x instead of velocity v=x/t nothing would change except that, not by calculation but by experimentation alone, it would have turned out that and L(m) is just a rotation in the euclidian space with no real valued eigenvectors. It is an interesting exercise to compare this result with the assumptions of reciprocity, homogeneity and isotropy mentioned earlier. Do the linear equations contained in the matrix formulation of the Lorentz transformation above assume a particular spacetime from the outset? independent of It is further important to mention that the determinant velocity. This directly implicates that spacetime volumes are conserved.

The geometry of space-time The discussion above was simplified by assuming that the symbols x,y,z,t were to be understood as intervals. The treatment given below is suitable for an undergraduate level of presentation. SR uses a ‘flat’ 4-dimensional Minkowski space, which is an example of a space-time. This space, however, is very similar to the standard 3 dimensional Euclidean space, and fortunately by that fact, very easy to work with. The differential of distance(ds) in cartesian 3D space is defined as:

where (dx1,dx2,dx3) are the differentials of the three spatial dimensions. In the geometry of special relativity, a fourth dimension, time, is added, with units of c, so that the equation for the differential of distance becomes:

In many situations it may be convenient to treat time as imaginary (e.g. it may simplify equations), in which case t in the above equation is replaced by i.t’, and the metric becomes

If we reduce the spatial dimensions to 2, so that we can represent the physics in a 3-D space

We see that things such as light which move at the speed of light lie along a dual-cone:

defined by the equation

or

Which is the equation of a circle with r=c*dt. The path of something that moves at the speed of light is known as a null geodesic. If we extend the equation above to three spatial dimensions, the null geodesics are continuous concentric spheres, with radius = distance = c×(±time).

This null dual-cone represents the “line of sight” of a point in space. That is, when we look at the stars and say “The light from that star which I am receiving is X years old.”, we are looking down this line of sight: a null geodesic. We are looking at an event meters away and d/c seconds in the past. For this reason the null dual cone is also known as the ‘light cone’. (The point in the lower left of the picture below represents the star, the origin represents the observer, and the line represents the null geodesic “line of sight”.)

The cone in the -t region is the information that the point is ‘receiving’, while the cone in the +t section is the information that the point is ‘sending’.

Length contraction, time dilation and phase Consider two inertial frames in standard configuration. There is a rigid rod moving along in the second frame at v m/s. The length of the rod is determined by observing the positions of the end points of the rod simultaneously - if the rod is moving it would be nonsense to use any other measure of length. An observer who is moving at the same velocity as the rod measures its “rest length”. The Lorentz transformation for coordinates along the x axis is: x’ = γ(x − vt)

Suppose the positions, x1,x2, of the two ends of the rod are determined simultaneously (ie: t is constant):

Or, using for the rest length of the rod and L = (x1 − x2) for the length of the rod that is measured by the observer who sees it fly past at v m/s:

Or, elaborating γ:

In other words the length of an object moving with velocity v is contracted in the direction of motion by a factor

in the direction of motion.

The Lorentz transformation also affects the rate at which clocks appear to change their readings. The Lorentz transformation for time is: t’ = γ(t − vx / c2) This transformation has two components: t’ = γt − γvx / c2 and is a straight line graph (ie: t’ = mt + c). The gradient of the graph is γ so: ∆t’ = γ∆t or:

Therefore clocks in the moving frame will appear to go slow, if T0 is a time interval in the rest frame and T is a time interval in the moving frame: T = γT0 Or, expanding:

The intercept of the graph is: − γvx / c2 This means that if a clock at point x is compared with a clock that was synchronised between frames at the origin it will show a constant time difference of γvx / c2 seconds. This quantity is known as the relativistic phase difference or “phase”.

The relativistic phase is as important as the length contraction and time dilation results. It is the amount by which clocks that are synchronised at the origin go out of synchronisation with distance along the direction of travel. Phase affects all clocks except those at the point where clocks are syncronised and the infinitessimal y and z planes that cut this point. All clocks everywhere else will be out of synchronisation between the frames. The effect of phase is shown in the illustration below:

If the inertial frames are each composed of arrays of clocks spread over space then the clocks will be out of synchronisation as shown in the illustration above.

Hyperbolic geometry In the flat spacetime of Special Relativity: s2 = c2t2 − x2 − y2 − z2 Considering the x-axis alone: s2 = c2t2 − x2 The standard equation of a hyperbola is:

In the case of spacetime:

Spacetime intervals separate one place or event in spacetime from another. So, for a given motion from one place to another or a given fixed length in one reference frame, given time interval etc. the metric of spacetime describes a hyperbolic space. This hyperbolic space encompasses the coordinates of all the observations made of the given interval by any observers.

It is possible to conceive of rotations in hyperbolic space in a similar way to rotations in Euclidean space. The idea of a rotation in hyperbolic space is summarised in the illustration below:

A rotation in hyperbolic space is equivalent to changing from one frame of reference to another whilst observing the same spacetime interval. It is moving from coordinates that give: (ct)2 − x2 = s2 to coordinates that give:

The formula for a rotation in hyperbolic space provides an alternative form of the Lorentz transformation ie:

From which: x = x’coshφ + ct’sinhφ ct = x’sinhφ + ct’coshφ The value of φ can be determined by considering the coordinates assigned to a moving light that moves along the x axis from the origin at vmsec1 flashes on for t seconds then flashes off. The coordinates assigned by an observer on the light are: t’,0,0,0, the coordinates assigned by the stationary observer are t,x = vt,0,0. The hyperbola representing these observations is illustrated below:

The equation of the hyperbola is: (ct)2 − x2 = s2 = (ct’)2 but x=vt for the end of the flash so:

Now, from hyperbolic trigonometry:

But

so:

and, from the hyperbolic trigonometric formula sinhφ = tanhφcoshφ:

Inserting these values into the equations for the hyperbolic rotation: x = x’coshφ + ct’sinhφ x = γx’ + ct’γv / c Which gives the standard transform for x: x = γ(x’ + vt’) In a similar way ct = x’sinhφ + ct’coshφ is equivalent to: t = γ(t’ + vx / c2) So the Lorentz transformations can also be derived from the assumption that boosts are equivalent to rotations in hyperbolic space with a metric s2 = c2t2 − x2 − y2 − z2. The quantity φ is known as the rapidity of the boost.

Addition of velocities Suppose there are three observers 1, 2, and 3 who are moving at different velocities along the x-axis. Observers 1 and 2 are moving at a relative velocity v and observers 2 and 3 are moving at a relative velocity of u’. The problem is to determine the velocity of observer 3 as observed by observer 1 (u).

It turns out that there is a very convenient relationship between rapidities that solves this problem: If v / c = tanhφ and u’ / c = tanhα then: u / c = tanh(φ + α) In other words the rapidities can be simply added from one observer to another ie: σ=φ+α Hence: tanh(σ) = tanh(φ + α) So the velocities can be added by simply adding the rapidities. Using hyperbolic trigonometry:

Therefore:

Which is the relativistic velocity addition theorem. The relationship u / c = tanh(φ + α) is shown below:

Velocity transformations can be obtained without referring to the rapidity. The general case of the transformation of velocities in any direction is derived as follows:

where

etc. are the components of the velocity in the x, y, z directions.

Writing out the components of velocity:

But from the Lorentz transformations: dx’ = γ(dx − vdt) dy’ = dy dz’ = dz

dt’ = γ(dt − vdx / c2) Therefore:

Dividing top and bottom of each fraction by dt:

Substituting

The full velocity transformations are tabulated below:

Having calculated the components of the velocity vector it it now possible to calculate the magnitudes of the overall vectors between frames:

Addition of velocities - another approach In classical physics, velocities simply add. If an object moves with speed u in one reference frame, which is itself moving at v with respect to a second frame, the object moves at speed u+v in that second frame. This is inconsistant with relativity because it predicts that if the speed of light is c in the first frame it will be v+c in the second. We need to find an alternative formula for combining velocities. We can do this with the Lorentz transform. Because the factor v/c will keep recurring we shall call that ratio β. We are considering three frames; frame O, frame O’ which moves at speed u with respect to frame O, and frame O” which moves at speed v with respect to frame O’. We want to know the speed of O” with respect to frame O,U which would classically be u+v. The transforms from O to O’ and O’ to O” can be written as matrix equations,

where we are defining the β’s and γ’s as

We can combine these to get the relationship between the O and O” coordinates simply by multiplying the matrices, giving

This should be the same as the Lorentz transform between the two frames,

These two sets of equations do look similar. We can make them look more similar still by taking a factor of 1+ββ’ out of the matrix in (1) giving#

This will be identical with equation 2 if

Since the two equations must give identical results, we know these conditions must be true. Writing the β’s in terms of the velocities equation 3a becomes

which tells us U in terms of u and v. A little algebra shows that this implies equation 3b is also true Multiplying by c we can finally write.

Notice that if u or v is much smaller than c the denominator is approximately 1, and the velocities approximately add but if either u or v is c then so is U, just as we expected.

Acceleration transformation It was seen above that: u / c = tanhφ and, if v / c = tanhα and u’ / c = tanhε the velocity addition theorem can be expressed as the sum of the rapidities: φ=α+ε If we differentiate this equation with respect to t to investigate acceleration, then assuming v is constant:

(1)

But

is also equal to:

But φ = tanh

and hence:

But:

−1

(u / c) and the derivative of an arctangent is given by:

ie: γ(u) is gamma for observers moving at a relative velocity of u. So:

But from the length contraction formula:

Therefore, substituting these two equations in (1):

Applying the differential of arctanh as before to determine

:

This is a different result from the Newtonian formula in which du / dt = du’ / dt’. The proper acceleration, α is defined as the acceleration of an object in its rest frame. It is the instantaneous change in velocity for an observer for whom u’ = 0 and α = du’ / dt’. In these circumstances:

Another approach to acceleration Classically we would talk about a particle at x(t) with acceleration dx²/d²t. In relativity we must treat time as just another coordinate, and use derivatives with respect to proper time, which means our notion of acceleration will change. We can assume the particle is moving slower than light.

Since τ increases monotonically with t (no time travel allowed) we can easily parameterise the path of the particle with τ rather than t. Derivatives with respect to these two variables then differ only by a factor of γ

which gives us the connection between the classical and relativstic formulae. Lets suppose we have a particle moving on the path (x(τ),ct(τ)), where τ is the particle’s proper time. Its velocity four vector is

In the particles rest frame this is (0,c), which has constant magnitude -c2, but this scalar must be the same in all frames, thus

That is, the magnitude of the velocity is always constant. Differentiating this, we can immediately say

so, velocity and acceleration are always perpendicular. These two results are much simpler than in classical physics. Differentiating the velocity gives us, after some algebra ,

when the motion is along the x-axis. Since the spatial and temporal components must be 3-vectors and scalars respectively we immediately see that for motion in arbitary directions,

The magnitude of this is

which is the classical magnitude, corrected by a factor dependant on γ. When the velocity is much less than c this factor is approximately 1, so in that limit the magnitudes are the same, as they should be. Knowing this, we can work out the equation of motion for a particle with constant acceleration a along the x-axis. For simplicity, we’ll assume the initial velocity is 0. Since the accleration is constant

Integrating once we get

or, on rearranging

If at is much less than c this gives a velocity of approximately at, identical to the classical result, but as t tends to infinity the velocity tends to c. The position is

where d is some constant. We can choose our coordinate system to make d zero.

Calling the interval between the origin and the particle, I, we have

Thus the interval between the particle and the origin is constant. Notice, this is the equation for an hyperbola, so we know the particle’s trajectory.

Momentum Classically, momentum is velocity multiplied by mass. We can use the same definition in relativity, and see where it takes us.

You may sometimes see the product m0γ which shows up here called the relativistic mass, but we will not be using this approach. The mass, m0, is generally called the rest mass, to distinguish it from the relativistic mass. The spatial component of the four-momemntum is clearly the classical momentum, scaled by a factor of γ. At speeds much less than c this will be approximately 1. The temporal component is m0γc. To see what this means we can look at its value when v/c is small.

The first term in this expansion is a constant. The second term is

which we recognise as being the classical kinetic energy, divided by c.

Now, adding a constant to the definition of kinetic energy makes no real difference, since all that matters are changes in energy, so we can identify this temporal component of relativistic momentum with the energy over c.

We then have

Even at rest, the particle has a kinetic energy,

the most famous relativistic equation.

Force Classically, we have

We can get the equivalent relativistic equation simply by replacing 3-vectors with 4-vectors and t with τ, giving

Provided the rest mass is constant, as it is for all simple systems, we can rewrite this as

We already know about a so we can now write

The temporal component of this is essentially the power, the rate of change of energy with time, as might be expected from energy being the temporal component of momentum.

Accelerated Frames and Event Horizons

Figure: Spacetime diagram showing the world line of the origin of a reference frame undergoing constant acceleration. We know this world line is an hyperbola, with asymptotes x=±ct. Looking at the diagram show another property. No light from the ‘twilight zone’, where x is smaller than ct, can ever reach the accelerated observer. Hence, no event in the twilight zone can affect the observer. Conversely, there is a region, where x is smaller than -ct, which no light from the accelerated observer can ever reach. Hence the observer can not affect any events in this region. Because of this we call the two aymptotes event horizons. In some ways, they are like one-way barriers in space-time. The accelerated observer can see objects vanishing behind an event horizon, but they never see any reemerge from it. These particular event horizons are observer dependent, as are all event horizons in special relativity. However, general relativity permits eventhorizons which are observer independent. We are not going to prove this, but we can some idea about how event horizons apply to general relativity by using its basic principle, called the equivalence principle by Einstein: that gravity is nothing more than an inertial force. This means that gravity is locally indistinguishable from forces due to acceleration; if the elevator doors are closed, we can’t tell whether we are stationary in a 1g gravity field or being accelerated at 1g in zero gravity.

This suggests that something can be learned about general relativity by examining the properties of accelerated reference frames Now, since the gravitational force on the Earth points downward, it follows that we must be constantly accelerating upward as we stand on the surface of the Earth. The obvious problem with this interpretation of gravity is that we don’t appear to be moving away from the center of the Earth, which would seem to be a natural consequence of such an acceleration. However, the calculation of the previous chapter shows that even though the object associated with the curved world line in the figure above is accelerating away from the origin, it always remains the same distance (in its own frame) from the origin. In other words, even though we are accelerating away from the center of the Earth, the distance to the center of the Earth remains constant! Therefore, because of the equivalence principle, we can expect to see the same things as an accelerated observer in special relativity, including an event horizon. For acceleration in special relativity, inertial objects pass the event horizon at speed c at time infinity, relative to the observer. Similarly, in general relativity, freely falling objects pass the event horizon at speed c at time infinity, relative to a static observer. By reversing the trajectories we can conclude that at the event horizon the escape velocity is c, i.e gravity is so strong light can not escape. Thus, using the equivalence principle, and behaviour under acceleration in special relativity, we’ve predicted black holes. There isn’t actually an event horizon inside the earth, since the mass of the earth is not concentrated in a point and the earth can only be treated as a point mass while outside the earth’s surface, which lies outside where the event horizon should otherwise have been.

The red shift

Spacetime diagram for explaining the gravitational red shift. Light emitted at a lower level in a gravitational field has its frequency reduced as it travels to a higher level. This phenomenon is called the gravitational red shift. We can see why this happens by using the principle of equivalence. Being in a gravitational field is equivalent to being in an accelerated frame, so knowing how the doppler shift works in such a frame will tell us how it works in a gravitational field. We view the process of light unaccelerated or inertial frame, reference frame the observer of indicated by the curved red world force to the left.

emission and absorption from the the as shown in the figure above. In this the light is accelerating to the right, as line, which is equivalent to a gravitational

The light is emitted at point A with frequency ω by a source which is stationary at this instant. At this instant the observer is also stationary in this frame. However, by the time the light gets to the observer, they have a velocity to the right which means that the observer measures a Doppler shifted freqency ω’ for the light. Since the observer is moving away from the source, ω’ < ω, as indicated above. The relativistic Doppler shift is given by

so we need to compute U/c. The line of simultaneity for the observer at point B goes through the origin, and is thus given by line segment OB. The slope of this line is U/c, where U is the velocity of the observer at point B. From the figure we see that this slope is also given by the ratio X’ /X. Equating these, eliminating X in favor of L = √(X2 - X′2), which is the actual invariant distance of the observer from the origin, and substituting into the previous equation results in our gravitational red shift formula:

If X′ = 0, then there is no redshift, because the source is collocated with the observer. On the other hand, if the source is located at the origin, so X′=X, the Doppler shifted frequency is zero. In addition, the light never gets to the observer, since the world line is asymptotic to the light world line passing through the origin. If the source is at a higher level in the gravitational field than the observer, so that X′ < 0, then the frequency is shifted to a higher value, i. e., it becomes a blue shift. To see how this doppler shift relates to the strength of gravity, g, and the distance h between the source and the observer, first note that

Making these substitutions gives

So the redshift is proportional to gravity. Since this doppler shift doesn’t depend on the type of wave we can conclude that it is actually caused by time dilation, just like the doppler shift due to relative motion. That is, gravity slows down time.

Energy and frequency In equation 1 gh is the change in gravitational potential energy so the change in frequency is proportional to change in potential energy, which suggests there might be a connection with energy conservation.

However, since we haven’t yet established any connection between frequency and energy, we can’t simply apply a energy conservation argument. Instead, we can argue in reverse, finding out what the energyfrequency relationship must be is energy is to be conserved. Suppose we have two identical systems, both at rest in a uniform gravitational field g with initial energy E separated by a vertical distance h. The system has mass E/c2, giving it potential energy, so the total energy of the two systems is initially

where the second term is due to the lesser potential energy of the lower system. The lower system emits a burst of waves, frequency ω, energy E(ω). When this reaches the upper system the waves have been red-shifted to frequency ω’, energy E(ω’). This energy is absorbed by the upper system. The total energy is now

Since we want to preserve energy conservation, these two equations must give the same result. Equating them, we get

Comparing this with the doppler shift we see that

which can only be true if E(ω) is proportional to ω I.e, energy conservation implies energy is proportional to frequency, which is one of the axioms of quantum theory. We could equally well have started with the quantum result and proved the gravitational red shift must exist. Either theory requires the other for consistency.

Since energy and frequency are each the temporal components of a fourvector, their being proportional implies the four-vectors themselves, and their spatial components, are also proportional. So, for waves, momentum is proportional to k. Remember too, we saw earlier, when we looked at Hamilton’s equations, that classical mechanics would be equivalent to a theory of anisotropic waves, in the geometrical optics limit, if energy were proportional to frequency and momentum to wave number. This proportionality isn’t just required for energy conservation, it would make possible a theory uniting waves and particles. None of this actually proves the proportionality, doing that requires experiment, but it does make it a natural assumptiom, which is indeed confirmed by experiment. Because of all this, from now on we’ll assume that energy and frequency are related in this way, with the constant of proportionality being

Gravity and curvature The gravitational redshift also implies that space is curved. We can see this by considering a rectangle in space-time. Without gravity, if we start at some point A, wait for time t, then move at light-speed to the right for a distance h, we get to the same place, B, as if we move at light-speed for a distance h then wait for time t at rest with respect to A. With gravity, if we follow the first path, we rise a distance of ct then wait for time t. On the second path, we begin by waiting for time t, but this is dilated by gravity. To an observer at B we appear to be waiting for a time t(1+gh/c2) before we start, so we end up at B later than on the first path. Thus, with gravity, it matters which order we add distance vectors in. This can’t happen if space is flat, so space must be curved. To describe how it’s curved we’d need the techniques of General Relativity.

Potential Momentum In classical physics we know that kinematics can often be described by a potential energy alone. Now we’ve seen that in relativity the energy is just the temporal component of the momentum 4-vector, so we should expect

the same of the potential energy. To see how this works, we’ll reason by analogy from the classical case. For a free, non-relativistic particle of mass m, the total energy E equals the kinetic energy K and is related to the momentum Π of the particle by

In the non-relativistic case, the momentum is Π= mv, where v is the particle velocity. If the particle is not free, but is subject to forces associated with a potential energy U(x,y,z), then the equation must be modified to account for the contribution of U to the total energy:

The force on the particle is related to the potential energy by

For a free, relativistic particle, we have

The obvious way to add forces to the relativistic case is by rewriting this equation with a potential energy:

However is a four-vector, so an equation with something subtracted from just one of the components of this four-vector is not relativistically invariant. In other words, this equation doesn’t obey the principle of relativity, and therefore cannot be correct! How can we fix this problem? One way is to define a new four-vector with U/c being its timelike part and some new vector Q being its spacelike part:

We then subtract Q from the momentum Π. When we do this, equation (13.5) becomes

The quantity Q is called the potential momentum and Q is the potential fourmomentum. If |Π-Q| is much smaller than mc, this becomes approximately

This expression for the energy has the same form as the Hamiltonian we looked at for classical velocity dependent forces, so we know it predicts a force perpendicular to the velocity, when the condition is met. It turns out to be perpendicular even when the condition is met. In classical physics the potential momentum is an optional extra. In relativity it is a necessary part of any potential field. Some additional terminology is useful. We define

as the kinetic momentum since in the classical case it reduces to mv. In order to avoid confusion, we rename Π the total momentum. Thus, the total momentum equals the kinetic plus the potential momentum, in analogy with energy.

Conservation of 4 momentum We earlier introduced the ideas of energy and momentum conservation. In other words, if we have a number of particles isolated from the rest of the universe, each with momentum pi and energy Ei, then particles may be created and destroyed and they may collide with each other. In these interactions the energy and momentum of each particle may change, but the sum total of all the energy and the sum total of all the momentum remains constant with time:

The expression is simpler in terms of four-momentum:

At this point a statement such as the one above should ring alarm bells. Just what does it mean to say that the total energy and momentum remain constant with time in the context of relativity? Which time? The time in which reference frame? This figure illustrates the problem.

Suppose two particles exchange four-momentum remotely at the time indicated by the fat horizontal bar in the left panel. Conservation of fourmomentum implies that

where the subscripted letters correspond to the particle labels in the figure. Primed values refer to the momentum after the exchange while no primes indicates values before the exchange. Now view the exchange from the reference frame in the right panel. A problem with four-momentum conservation exists in the region between the thin horizontal lines. In this region particle B has already transferred its fourmomentum, but it has yet to be received by particle A. In other words, fourmomentum is not conserved in this reference frame! This problem is so serious that we must eliminate the concept of action at a distance from the repertoire of physics. The only way to have particles interact remotely and still conserve four-momentum in all reference frames is to assume that all remote interactions are mediated by another particle, or by a field.

If the force is being mediated by a particle then first, particle A emits particle C in a manner which conserves the four-momentum. Second, particle C is absorbed by particle B in a similarly conservative interaction. If the force is being mediated by a field then first, particle A emits wave C, with momentum proportional to its wavenumber in a manner, which conserves the four-momentum. Wave C then travels at c or less until it is absorbed by particle B in a similarly conservative interaction. We’ll see that in quantum theory the difference between a particle and a field vanishes, so these two pictures actually both describe the same mechanism a different perspective. Which every picture we use, four-momentum is conserved at all times in all reference frames. In other words, momentum and energy are transferred from particle A to particle B in a two step process. In between the momentum resides in a particle or field

Mathematical Appendix Mathematics of the Lorentz Transformation Equations Consider two observers O and O’, moving at velocity relative to each other, who observe the same event such as a flash of light. How will the coordinates recorded by the two observers be interrelated? These can be derived using linear algebra on the basis of the postulates of relativity and an extra homogeneity and isotropy assumption. The homogeneity and isotropy assumption: space is uniform and homogenous in all directions. If this were not the case then when comparing lengths between coordinate systems the lengths would depend upon the position of the measurement. For instance, if two points would depend upon position.

the distance between

The linear equations relating coordinates in the primed and unprimed frames are:

There is no relative motion in the y or z directions so, according to the ‘relativity’ postulate:

Hence:

and and So the following equations remain to be solved:

If space is isotropic (the same in all directions) then the motion of clocks should be independent of the y and z axes (otherwise clocks placed symmetrically around the x-axis would appear to disagree. Hence

so:

Events satisfying

must also satisfy

. So:

and

Given that the equations are linear then

and:

and

Therefore the correct transformation equation for

is:

The analysis to date gives the following equations:

Assuming that the speed of light is constant, the coordinates of a flash of light that expands as a sphere will satisfy the following equations in each coordinate system:

Substituting the coordinate transformation equations into equation gives:

rearranging:

We demanding that this is equivalent with

So we get:

Solving these 3 simultaneous equations gives:

Substituting these values into:

the second

gives:

The inverse transformation is: